2.03c Conditional probability: using diagrams/tables

341 questions

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Edexcel S1 2016 January Q4
13 marks Moderate -0.3
4. A training agency awards a certificate to each student who passes a test while completing a course.
Students failing the test will attempt the test again up to 3 more times, and, if they pass the test, will be awarded a certificate.
The probability of passing the test at the first attempt is 0.7 , but the probability of passing reduces by 0.2 at each attempt.
  1. Complete the tree diagram below to show this information. \includegraphics[max width=\textwidth, alt={}, center]{70137e9a-0a6b-48b5-8dd4-c436cb063351-08_545_1244_639_340} A student who completed the course is selected at random.
  2. Find the probability that the student was awarded a certificate.
  3. Given that the student was awarded a certificate, find the probability that the student passed on the first or second attempt. The training agency decides to alter the test taken by the students while completing the course, but will not allow more than 2 attempts. The agency requires the probability of passing the test at the first attempt to be \(p\), and the probability of passing the test at the second attempt to be ( \(p - 0.2\) ). The percentage of students who complete the course and are awarded a certificate is to be \(95 \%\)
  4. Show that \(p\) satisfies the equation $$p ^ { 2 } - 2.2 p + 1.15 = 0$$
  5. Hence find the value of \(p\), giving your answer to 3 decimal places. \includegraphics[max width=\textwidth, alt={}, center]{70137e9a-0a6b-48b5-8dd4-c436cb063351-09_2261_47_313_37}
Edexcel S1 2017 January Q2
9 marks Easy -1.2
An integer is selected at random from the integers 1 to 50 inclusive. \(A\) is the event that the integer selected is prime. \(B\) is the event that the integer selected ends in a 3 \(C\) is the event that the integer selected is greater than 20
The Venn diagram shows the number of integers in each region for the events \(A , B\) and \(C\) \includegraphics[max width=\textwidth, alt={}, center]{1130517e-33d0-41b1-9303-2d981379954d-04_607_1125_593_413}
  1. Describe in words the event \(( A \cap B )\)
  2. Write down the probability that the integer selected is prime.
  3. Find \(\mathrm { P } \left( [ A \cup B \cup C ] ^ { \prime } \right)\) Given that the integer selected is greater than 20
  4. find the probability that it is prime. Using your answers to (b) and (d),
  5. state, with a reason, whether or not the events \(A\) and \(C\) are statistically independent. Given that the integer selected is greater than 20 and prime,
  6. find the probability that it ends in a 3
Edexcel S1 2017 January Q5
6 marks Moderate -0.3
  1. In a survey, people were asked if they use a computer every day.
Of those people under 50 years old, \(80 \%\) said they use computer every day. Of those people aged 50 or more, \(55 \%\) said they use computer every day. The proportion of people in the survey under 50 years old is \(p\)
  1. Draw a tree diagram to represent this information. In the survey, 70\% of all people said they use computer every day.
  2. Find the value of \(p\) One person is selected at random. Given that this person uses a computer every day,
  3. find the probability that this person is under 50 years old.
    \href{http://PhysicsAndMathsTutor.com}{PhysicsAndMathsTutor.com}
Edexcel S1 2018 January Q2
8 marks Moderate -0.8
2.
  1. Shade the region representing the event \(A \cup B ^ { \prime }\) on the Venn diagram below. \includegraphics[max width=\textwidth, alt={}, center]{01259350-0119-4500-a81b-bfa1b4234559-06_355_563_306_694} The two events \(C\) and \(D\) are mutually exclusive.
    Given that \(\mathrm { P } ( C ) = \frac { 1 } { 5 }\) and \(\mathrm { P } ( D ) = \frac { 3 } { 10 }\) find
    1. \(\quad \mathrm { P } ( C \cup D )\)
    2. \(\mathrm { P } ( C \mid D )\) The two events \(F\) and \(G\) are independent.
      Given that \(\mathrm { P } ( F ) = \frac { 1 } { 6 }\) and \(\mathrm { P } ( F \cup G ) = \frac { 3 } { 8 }\) find
    1. \(\mathrm { P } ( G )\)
    2. \(\mathrm { P } \left( F \mid G ^ { \prime } \right)\)
Edexcel S1 2018 January Q6
11 marks Standard +0.3
  1. Anju has a bag that contains 5 socks of which 2 are blue.
Anju randomly selects socks from the bag, one sock at a time. She does not replace any socks but continues to select socks at random until she has both blue socks. The discrete random variable \(S\) represents the total number of socks that Anju has selected.
  1. Write down the value of \(\mathrm { P } ( S = 1 )\)
  2. Find \(\mathrm { P } ( S > 2 )\)
  3. Find \(\mathrm { P } ( S = 3 )\)
  4. Given that the second sock selected is blue, find the probability that Anju selects exactly 3 socks.
  5. Find \(\mathrm { P } ( S = 5 )\)
Edexcel S1 2019 January Q1
9 marks Standard +0.3
The Venn diagram shows the probability of a randomly selected student from a school being in the sets \(L , B\) and \(C\), where \(L\) represents the event that the student has instrumental music lessons \(B\) represents the event that the student plays in the school band \(C\) represents the event that the student sings in the school choir \(p , q , r\) and \(s\) are probabilities. \includegraphics[max width=\textwidth, alt={}, center]{d3f4450d-60eb-49b6-be1b-d2fcfad0451f-02_504_750_735_598}
  1. Select a pair of mutually exclusive events from \(L , B\) and \(C\). Given that \(\mathrm { P } ( L ) = 0.4 , \mathrm { P } ( B ) = 0.13 , \mathrm { P } ( C ) = 0.3\) and the events \(L\) and \(C\) are independent,
  2. find the value of \(p\),
  3. find the value of \(q\), the value of \(r\) and the value of \(s\). A student is selected at random from those who play in the school band or sing in the school choir.
  4. Find the exact probability that this student has instrumental music lessons.
Edexcel S1 2021 January Q1
5 marks Moderate -0.8
  1. The Venn diagram shows the events \(A , B\) and \(C\) and their associated probabilities. \includegraphics[max width=\textwidth, alt={}, center]{4f034b9a-94c8-42f2-bd77-9adec277aba6-02_584_1061_296_445}
Find
  1. \(\mathrm { P } \left( B ^ { \prime } \right)\)
  2. \(\mathrm { P } ( A \cup C )\)
  3. \(\mathrm { P } \left( A \mid B ^ { \prime } \right)\)
Edexcel S1 2023 January Q2
10 marks Moderate -0.3
  1. Two bags, \(\boldsymbol { X }\) and \(\boldsymbol { Y }\), each contain green marbles (G) and blue marbles (B) only.
  • Bag \(\boldsymbol { X }\) contains 5 green marbles and 4 blue marbles
  • Bag \(\boldsymbol { Y }\) contains 6 green marbles and 5 blue marbles
A marble is selected at random from bag \(\boldsymbol { X }\) and placed in bag \(\boldsymbol { Y }\) A second marble is selected at random from bag \(\boldsymbol { X }\) and placed in bag \(\boldsymbol { Y }\) A third marble is then selected, this time from bag \(\boldsymbol { Y }\)
  1. Use this information to complete the tree diagram shown on page 7
  2. Find the probability that the 2 marbles selected from bag \(\boldsymbol { X }\) are of different colours.
  3. Find the probability that all 3 marbles selected are the same colour. Given that all three marbles selected are the same colour,
  4. find the probability that they are all green. 2nd Marble (from bag \(\boldsymbol { X }\) ) \section*{3rd Marble (from bag Y)} \begin{figure}[h]
    \captionsetup{labelformat=empty} \caption{1st Marble (from bag \(\boldsymbol { X }\) )} \includegraphics[alt={},max width=\textwidth]{c316fa29-dedc-4890-bd82-31eb0bb819f9-07_1694_1312_484_310}
    \end{figure}
Edexcel S1 2023 January Q4
13 marks Standard +0.3
  1. In the Venn diagram below, \(A\) and \(B\) represent events and \(p , q , r\) and \(s\) are probabilities. \includegraphics[max width=\textwidth, alt={}, center]{c316fa29-dedc-4890-bd82-31eb0bb819f9-12_400_789_347_639} $$\mathrm { P } ( A ) = \frac { 7 } { 25 } \quad \mathrm { P } ( B ) = \frac { 1 } { 5 } \quad \mathrm { P } \left[ \left( A \cap B ^ { \prime } \right) \cup \left( A ^ { \prime } \cap B \right) \right] = \frac { 8 } { 25 }$$
    1. Use algebra to show that \(2 p + 2 q + 2 r = \frac { 4 } { 5 }\)
    2. Find the value of \(p\), the value of \(q\), the value of \(r\) and the value of \(s\)
    3. Two events, \(C\) and \(D\), are such that $$\mathrm { P } ( C ) = \frac { x } { x + 5 } \quad \mathrm { P } ( D ) = \frac { 5 } { x }$$ where \(x\) is a positive constant.
      By considering \(\mathrm { P } ( C ) + \mathrm { P } ( D )\) show that \(C\) and \(D\) cannot be mutually exclusive.
Edexcel S1 2024 January Q3
8 marks Easy -1.3
  1. In a sixth form college each student in Year 12 and Year 13 is either left-handed (L) or right-handed (R).
The partially completed tree diagram, where \(p\) is a probability, gives information about these students. \includegraphics[max width=\textwidth, alt={}, center]{86446ce3-496a-4f02-9566-9b207bac9efa-10_960_981_477_543}
  1. Complete the tree diagram, in terms of \(p\) where necessary. The probability that a student is left-handed is 0.11
  2. Find the value of \(p\)
  3. Find the probability that a student selected at random is in Year 12 and left-handed. Given that a student is right-handed,
  4. find the probability that the student is in Year 12
Edexcel S1 2024 January Q6
9 marks Standard +0.3
  1. The events \(A\) and \(B\) satisfy
$$\mathrm { P } ( A ) = x \quad \mathrm { P } ( B ) = y \quad \mathrm { P } ( A \cup B ) = 0.65 \quad \mathrm { P } ( B \mid A ) = 0.3$$
  1. Show that $$14 x + 20 y = 13$$ The events \(B\) and \(C\) are mutually exclusive such that $$\mathrm { P } ( B \cup C ) = 0.85 \quad \mathrm { P } ( C ) = \frac { 1 } { 2 } x + y$$
    1. Find a second equation in \(x\) and \(y\)
    2. Hence find the value of \(x\) and the value of \(y\)
  2. Determine whether or not \(A\) and \(B\) are statistically independent. You must show your working clearly.
Edexcel S1 2004 January Q4
11 marks Moderate -0.8
4. \(\quad\) The events \(A\) and \(B\) are such that \(\mathrm { P } ( A ) = \frac { 2 } { 5 } , \mathrm { P } ( B ) = \frac { 1 } { 2 }\) and \(\mathrm { P } \left( A \quad B ^ { \prime } \right) = \frac { 4 } { 5 }\).
  1. Find
    1. \(\mathrm { P } \left( A \cap B ^ { \prime } \right)\),
    2. \(\mathrm { P } ( A \cap B )\),
    3. \(\mathrm { P } ( A \cup B )\),
    4. \(\mathrm { P } \left( \begin{array} { l l } A & B \end{array} \right)\).
  2. State, with a reason, whether or \(\operatorname { not } A\) and \(B\) are
    1. mutually exclusive,
    2. independent.
Edexcel AEA 2023 June Q5
21 marks Hard +2.3
5. \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{78ba3acc-4cca-4d15-8362-a27e425c5859-16_517_881_210_593} \captionsetup{labelformat=empty} \caption{Figure 2}
\end{figure} Figure 2 shows a partially completed Venn diagram of sports that a year group of students enjoy,where \(a , b , c , d\) and \(e\) are non-negative integers. The diagram shows how many students enjoy a combination of football( \(F\) ),golf( \(G\) ) and hockey \(( H )\) or none of these sports. There are \(n\) students in the year group.
It is known that
- \(\mathrm { P } ( F ) = \frac { 3 } { 7 }\) - \(\mathrm { P } ( H \mid G ) = \frac { 1 } { 3 }\) -\(F\) is independent of \(H \cap G\)
  1. Show that \(\mathrm { P } ( F \cap H \cap G ) = \frac { 1 } { 7 } \mathrm { P } ( G )\)
  2. Prove that if two events \(X\) and \(Y\) are independent,then \(X ^ { \prime }\) and \(Y\) are also independent.
  3. Hence find the value \(k\) such that \(\mathrm { P } \left( F ^ { \prime } \cap H \cap G \right) = k \mathrm { P } ( G )\)
  4. Show that \(c = \frac { 4 } { 3 } a\) Given further that \(\mathrm { P } ( F \mid H ) = \frac { 1 } { 5 }\)
  5. find an expression for \(d\) in terms of \(a\) ,and hence deduce the maximum possible value of \(a\) .
  6. Determine the possible values of \(n\) .
OCR S4 2016 June Q5
11 marks Standard +0.8
5 Events \(A\) and \(B\) are such that \(\mathrm { P } ( A ) = 0.5 , \mathrm { P } ( B ) = 0.6\) and \(\mathrm { P } \left( A \mid B ^ { \prime } \right) = 0.75\).
  1. Find \(\mathrm { P } ( A \cap B )\) and \(\mathrm { P } ( A \cup B )\).
  2. Determine, giving a reason in each case,
    1. whether \(A\) and \(B\) are mutually exclusive,
    2. whether \(A\) and \(B\) are independent.
    3. A further event \(C\) is such that \(\mathrm { P } ( A \cup B \cup C ) = 1\) and \(\mathrm { P } ( A \cap B \cap C ) = 0.05\). It is also given that \(\mathrm { P } \left( A \cap B ^ { \prime } \cap C \right) = \mathrm { P } \left( A ^ { \prime } \cap B \cap C \right) = x\) and \(\mathrm { P } \left( A \cap B ^ { \prime } \cap C ^ { \prime } \right) = 2 x\).
      Find \(\mathrm { P } ( C )\).
CAIE S1 2020 Specimen Q7
10 marks Standard +0.3
7 Bag \(A\) contains 4 balls numbered 2, 4, 5, 8. Bag \(B\) contains 5 balls numbered 1, 3, 6, 8, 8. Bag \(C\) contains 7 balls numbered \(2,7,8,8,8,8,9\). One ball is selected at random from each bag.
  • Event \(X\) is 'exactly two of the selected balls have the same number'.
  • Event \(Y\) is 'the ball selected from bag \(A\) has number 4'.
    1. Find \(\mathrm { P } ( X )\).
    2. Find \(\mathrm { P } ( X \cap Y )\) and hence determine whether or not events \(X\) and \(Y\) are independent.
    3. Find the probability that two balls are numbered 2, given that exactly two of the selected balls have the same number.
OCR S1 2011 June Q5
9 marks Moderate -0.8
5 A bag contains 4 blue discs and 6 red discs. Chloe takes a disc from the bag. If this disc is red, she takes 2 more discs. If not, she takes 1 more disc. Each disc is taken at random and no discs are replaced.
  1. Complete the probability tree diagram in your Answer Book, showing all the probabilities. \includegraphics[max width=\textwidth, alt={}, center]{48ffcd44-d933-40e0-818a-20d6db607298-4_730_1203_529_511} The total number of blue discs that Chloe takes is denoted by \(X\).
  2. Show that \(\mathrm { P } ( X = 1 ) = \frac { 3 } { 5 }\). The complete probability distribution of \(X\) is given below.
    \(x\)012
    \(\mathrm { P } ( X = x )\)\(\frac { 1 } { 6 }\)\(\frac { 3 } { 5 }\)\(\frac { 7 } { 30 }\)
  3. Calculate \(\mathrm { E } ( X )\) and \(\operatorname { Var } ( X )\).
OCR S1 2012 June Q4
7 marks Easy -1.2
4 A bag contains 5 red discs and 1 black disc. Tina takes two discs from the bag at random without replacement.
  1. The diagram shows part of a tree diagram to illustrate this situation. \section*{First disc}
    \includegraphics[max width=\textwidth, alt={}]{e23cb28b-49e5-436a-942d-e6320029c634-3_264_494_479_550}
    Complete the tree diagram in your Answer Book showing all the probabilities. \section*{Second disc}
  2. Find the probability that exactly one of the two discs is red. All the discs are replaced in the bag. Tony now takes three discs from the bag at random without replacement.
  3. Given that the first disc Tony takes is red, find the probability that the third disc Tony takes is also red.
OCR MEI S1 2012 January Q3
8 marks Standard +0.3
3 Jimmy and Alan are playing a tennis match against each other. The winner of the match is the first player to win three sets. Jimmy won the first set and Alan won the second set. For each of the remaining sets, the probability that Jimmy wins a set is
  • 0.7 if he won the previous set,
  • 0.4 if Alan won the previous set.
It is not possible to draw a set.
  1. Draw a probability tree diagram to illustrate the possible outcomes for each of the remaining sets.
  2. Find the probability that Alan wins the match.
  3. Find the probability that the match ends after exactly four sets have been played.
OCR MEI S1 2012 January Q4
6 marks Moderate -0.8
4 In a food survey, a large number of people are asked whether they like tomato soup, mushroom soup, both or neither. One of these people is selected at random.
  • \(T\) is the event that this person likes tomato soup.
  • \(M\) is the event that this person likes mushroom soup.
You are given that \(\mathrm { P } ( T ) = 0.55 , \mathrm { P } ( M ) = 0.33\) and \(\mathrm { P } ( T \mid M ) = 0.80\).
  1. Use this information to show that the events \(T\) and \(M\) are not independent.
  2. Find \(\mathrm { P } ( T \cap M )\).
  3. Draw a Venn diagram showing the events \(T\) and \(M\), and fill in the probability corresponding to each of the four regions of your diagram.
OCR MEI S1 2013 January Q3
8 marks Moderate -0.8
3 Each weekday Alan drives to work. On his journey, he goes over a level crossing. Sometimes he has to wait at the level crossing for a train to pass.
  • \(W\) is the event that Alan has to wait at the level crossing.
  • \(L\) is the event that Alan is late for work.
You are given that \(\mathrm { P } ( L \mid W ) = 0.4 , \mathrm { P } ( W ) = 0.07\) and \(\mathrm { P } ( L \cup W ) = 0.08\).
  1. Calculate \(\mathrm { P } ( L \cap W )\).
  2. Draw a Venn diagram, showing the events \(L\) and \(W\). Fill in the probability corresponding to each of the four regions of your diagram.
  3. Determine whether the events \(L\) and \(W\) are independent, explaining your method clearly.
OCR MEI S1 2009 June Q6
6 marks Moderate -0.8
6 Whitefly, blight and mosaic virus are three problems which can affect tomato plants. 100 tomato plants are examined for these problems. The numbers of plants with each type of problem are shown in the Venn diagram. 47 of the plants have none of the problems. \includegraphics[max width=\textwidth, alt={}, center]{3a5d18f5-b1fe-4513-ae4e-f37c20f172b5-3_668_812_998_664}
  1. One of the 100 plants is selected at random. Find the probability that this plant has
    (A) at most one of the problems,
    (B) exactly two of the problems.
  2. Three of the 100 plants are selected at random. Find the probability that all of them have at least one of the problems. Section B (36 marks)
OCR MEI S1 2009 June Q7
18 marks Easy -1.2
7 Laura frequently flies to business meetings and often finds that her flights are delayed. A flight may be delayed due to technical problems, weather problems or congestion problems, with probabilities \(0.2,0.15\) and 0.1 respectively. The tree diagram shows this information. \includegraphics[max width=\textwidth, alt={}, center]{3a5d18f5-b1fe-4513-ae4e-f37c20f172b5-4_608_1651_532_248}
  1. Write down the values of the probabilities \(a , b\) and \(c\) shown in the tree diagram. One of Laura's flights is selected at random.
  2. Find the probability that Laura's flight is not delayed and hence write down the probability that it is delayed.
  3. Find the probability that Laura's flight is delayed due to just one of the three problems.
  4. Given that Laura's flight is delayed, find the probability that the delay is due to just one of the three problems.
  5. Given that Laura's flight has no technical problems, find the probability that it is delayed.
  6. In a particular year, Laura has 110 flights. Find the expected number of flights that are delayed.
OCR MEI S1 2010 June Q7
18 marks Moderate -0.3
7 One train leaves a station each hour. The train is either on time or late. If the train is on time, the probability that the next train is on time is 0.95 . If the train is late, the probability that the next train is on time is 0.6 . On a particular day, the 0900 train is on time.
  1. Illustrate the possible outcomes for the 1000,1100 and 1200 trains on a probability tree diagram.
  2. Find the probability that
    (A) all three of these trains are on time,
    (B) just one of these three trains is on time,
    (C) the 1200 train is on time.
  3. Given that the 1200 train is on time, find the probability that the 1000 train is also on time. 3
  4. Write any calculations on page 5. \includegraphics[max width=\textwidth, alt={}, center]{091d6f43-ad01-4849-9f3c-3e58349aa169-4_2276_1490_324_363}
OCR MEI S1 2013 June Q7
18 marks Standard +0.3
7 Jenny has six darts. She throws darts, one at a time, aiming each at the bull's-eye. The probability that she hits the bull's-eye with her first dart is 0.1 . For any subsequent throw, the probability of hitting the bull's-eye is 0.2 if the previous dart hit the bull's-eye and 0.05 otherwise.
  1. Illustrate the possible outcomes for her first, second and third darts on a probability tree diagram.
  2. Find the probability that
    (A) she hits the bull's-eye with at least one of her first three darts,
    (B) she hits the bull's-eye with exactly one of her first three darts.
  3. Given that she hits the bull's-eye with at least one of her first three darts, find the probability that she hits the bull's-eye with exactly one of them. Jenny decides that, if she hits the bull's-eye with any of her first three darts, she will stop after throwing three darts. Otherwise she will throw all six darts.
  4. Find the probability that she hits the bull's-eye three times in total.
OCR MEI S1 2015 June Q2
5 marks Easy -1.3
2 A survey is being carried out into the sports viewing habits of people in a particular area. As part of the survey, 250 people are asked which of the following sports they have watched on television in the past month.
  • Football
  • Cycling
  • Rugby
The numbers of people who have watched these sports are shown in the Venn diagram. \includegraphics[max width=\textwidth, alt={}, center]{6015ae6c-bf76-4a0c-af0f-5c53f9c5ed2a-2_723_917_1183_575} One of the people is selected at random.
  1. Find the probability that this person has in the past month
    (A) watched cycling but not football,
    (B) watched either one or two of the three sports.
  2. Given that this person has watched cycling, find the probability that this person has not watched football.