2.03d Calculate conditional probability: from first principles

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OCR S4 2012 June Q1
5 marks Challenging +1.2
1 Independent random variables \(X\) and \(Y\) have distributions \(\mathrm { B } ( 7 , p )\) and \(\mathrm { B } ( 8 , p )\) respectively.
  1. Explain why \(X + Y \sim \mathrm {~B} ( 15 , p )\).
  2. Find \(\mathrm { P } ( X = 2 \mid X + Y = 5 )\).
OCR S4 2012 June Q8
5 marks Standard +0.8
8 Events \(A\) and \(B\) are such that \(\mathrm { P } ( A ) = 0.3\) and \(\mathrm { P } ( A \mid B ) = 0.6\).
  1. Show that \(\mathrm { P } ( B ) \leqslant 0.5\).
  2. Given also that \(\mathrm { P } ( A \cup B ) = x\), find \(\mathrm { P } ( B )\) in terms of \(x\).
OCR S4 2013 June Q7
13 marks Standard +0.3
7 Each question on a multiple-choice examination paper has \(n\) possible responses, only one of which is correct. Joni takes the paper and has probability \(p\), where \(0 < p < 1\), of knowing the correct response to any question, independently of any other. If she knows the correct response she will choose it, otherwise she will choose randomly from the \(n\) possibilities. The events \(K\) and \(A\) are 'Joni knows the correct response' and 'Joni answers correctly' respectively.
  1. Show that \(\mathrm { P } ( A ) = \frac { q + n p } { n }\), where \(q = 1 - p\).
  2. Find \(P ( K \mid A )\). A paper with 100 questions has \(n = 4\) and \(p = 0.5\). Each correct response scores 1 and each incorrect response scores - 1 .
  3. (a) Joni answers all the questions on the paper and scores 40 . How many questions did she answer correctly?
    (b) By finding the distribution of the number of correct answers, or otherwise, find the probability that Joni scores at least 40 on the paper using her strategy.
OCR S4 2014 June Q2
9 marks Standard +0.3
2 During an outbreak of a disease, it is known that \(68 \%\) of people do not have the disease. Of people with the disease, \(96 \%\) react positively to a test for diagnosing it, as do \(m \%\) of people who do not have the disease.
  1. In the case \(m = 8\), find the probability that a randomly chosen person has the disease, given that the person reacts positively to the test.
  2. What value of \(m\) would be required for the answer to part (i) to be 0.95 ?
Edexcel S1 2014 January Q7
9 marks Moderate -0.3
7. In a large college, \(\frac { 3 } { 5 }\) of the students are male, \(\frac { 3 } { 10 }\) of the students are left handed and \(\frac { 1 } { 5 }\) of the male students are left handed. A student is chosen at random.
  1. Given that the student is left handed, find the probability that the student is male.
  2. Given that the student is female, find the probability that she is left handed.
  3. Find the probability that the randomly chosen student is male and right handed. Two students are chosen at random.
  4. Find the probability that one student is left handed and one is right handed.
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 2016 January Q5
14 marks Standard +0.3
5. Rosie keeps bees. The amount of honey, in kg, produced by a hive of Rosie's bees in a season, is modelled by a normal distribution with a mean of 22 kg and a standard deviation of 10 kg .
  1. Find the probability that a hive of Rosie's bees produces less than 18 kg of honey in a season. The local bee keepers' club awards a certificate to every hive that produces more than 39 kg of honey in a season, and a medal to every hive that produces more than 50 kg in a season. Given that one of Rosie's bee hives is awarded a certificate
  2. find the probability that this hive is also awarded a medal.
    (5) Sam also keeps bees. The amount of honey, in kg, produced by a hive of Sam's bees in a season, is modelled by a normal distribution with mean \(\mu \mathrm { kg }\) and standard deviation \(\sigma \mathrm { kg }\). The probability that a hive of Sam's bees produces less than 28 kg of honey in a season is 0.8413 Only 20\% of Sam's bee hives produce less than 18 kg of honey in a season.
  3. Find the value of \(\mu\) and the value of \(\sigma\). Give your answers to 2 decimal places.
    (6)
    \includegraphics[max width=\textwidth, alt={}, center]{70137e9a-0a6b-48b5-8dd4-c436cb063351-11_2261_47_313_37}
Edexcel S1 2017 January Q2
9 marks Easy -1.2
  1. 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
  2. find the probability that it is prime. Using your answers to (b) and (d),
  3. 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,
  4. 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. (a) 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
(b) (i) \(\quad \mathrm { P } ( C \cup D )\) (ii) \(\mathrm { P } ( C \mid D )\) 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
(c) (i) \(\mathrm { P } ( G )\) (ii) \(\mathrm { P } \left( F \mid G ^ { \prime } \right)\)
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
  1. 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 2019 January Q3
10 marks Standard +0.3
  1. The weights of women boxers in a tournament are normally distributed with mean 64 kg and standard deviation 8 kg .
    1. Find the probability that a randomly chosen woman boxer in the tournament weighs less than 51 kg .
    In the tournament, women boxers who weigh less than 51 kg are classified as lightweight. Ren weighs 49 kg and she has a match against another randomly selected, lightweight woman boxer.
  2. Find the probability that Ren weighs less than the other boxer. In the tournament, women boxers who weigh more than \(H \mathrm {~kg}\) are classified as heavyweight. Given that \(10 \%\) of the women boxers in the tournament are classified as heavyweight,
  3. find the value of \(H\).
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 2021 January Q3
13 marks Moderate -0.3
3. The weights of packages that arrive at a factory are normally distributed with a mean of 18 kg and a standard deviation of 5.4 kg
  1. Find the probability that a randomly selected package weighs less than 10 kg The heaviest 15\% of packages are moved around the factory by Jemima using a forklift truck.
  2. Find the weight, in kg , of the lightest of these packages that Jemima will move. One of the packages not moved by Jemima is selected at random.
  3. Find the probability that it weighs more than 18 kg A delivery of 4 packages is made to the factory. The weights of the packages are independent.
  4. Find the probability that exactly 2 of them will be moved by Jemima.
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 \(s\) (ii) 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 2023 January Q5
17 marks Moderate -0.3
  1. The lengths, \(L \mathrm {~mm}\), of housefly wings are normally distributed with \(L \sim \mathrm {~N} \left( 4.5,0.4 ^ { 2 } \right)\)
    1. Find the probability that a randomly selected housefly has a wing length of less than 3.86 mm .
    2. Find
      1. the upper quartile ( \(Q _ { 3 }\) ) of \(L\)
      2. the lower quartile ( \(Q _ { 1 }\) ) of \(L\)
    A value that is greater than \(Q _ { 3 } + 1.5 \times \left( Q _ { 3 } - Q _ { 1 } \right)\) or smaller than \(Q _ { 1 } - 1.5 \times \left( Q _ { 3 } - Q _ { 1 } \right)\) is defined as an outlier.
  2. Find these two outlier limits. A housefly is selected at random.
  3. Using standardisation, show that the probability that this housefly is not an outlier is 0.993 to 3 decimal places. Given that this housefly is not an outlier,
  4. showing your working, find the probability that the wing length of this housefly is greater than 5 mm .
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 2014 June Q6
12 marks Moderate -0.3
6. The Venn diagram below shows the probabilities of customers having various combinations of a starter, main course or dessert at Polly's restaurant. \(S =\) the event a customer has a starter. \(M =\) the event a customer has a main course. \(D =\) the event a customer has a dessert. \includegraphics[max width=\textwidth, alt={}, center]{fa0dbe16-ace8-4c44-8404-2bc4e1879d57-10_602_1125_607_413} Given that the events \(S\) and \(D\) are statistically independent
  1. find the value of \(p\).
  2. Hence find the value of \(q\).
  3. Find
    1. \(\quad\) P( \(D \mid M \cap S\) )
    2. \(\operatorname { P } \left( D \mid M \cap S ^ { \prime } \right)\) One evening 63 customers are booked into Polly's restaurant for an office party. Polly has asked for their starter and main course orders before they arrive. Of these 63 customers 27 ordered a main course and a starter, 36 ordered a main course without a starter.
  4. Estimate the number of desserts that these 63 customers will have.
Edexcel S1 2015 June Q3
8 marks Easy -1.3
  1. A company employs 90 administrators. The length of time that they have been employed by the company and their gender are summarised in the table below.
Length of time employed, \(x\) yearsFemaleMale
\(x < 4\)916
\(4 \leqslant x < 10\)1420
\(10 \leqslant x\)724
One of the 90 administrators is selected at random.
  1. Find the probability that the administrator is female.
  2. Given that the administrator has been employed by the company for less than 4 years, find the probability that this administrator is male.
  3. Given that the administrator has been employed by the company for less than 10 years, find the probability that this administrator is male.
  4. State, with a reason, whether or not the event 'selecting a male' is independent of the event 'selecting an administrator who has been employed by the company for less than 4 years'.
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 \(\mathrm { P } ( C )\).