Questions — CAIE S2 (717 questions)

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CAIE S2 2010 November Q4
4 The masses, in milligrams, of three minerals found in 1 tonne of a certain kind of rock are modelled by three independent random variables \(P , Q\) and \(R\), where \(P \sim \mathrm {~N} \left( 46,19 ^ { 2 } \right) , Q \sim \mathrm {~N} \left( 53,23 ^ { 2 } \right)\) and \(R \sim \mathrm {~N} \left( 25,10 ^ { 2 } \right)\). The total value of the minerals found in 1 tonne of rock is modelled by the random variable \(V\), where \(V = P + Q + 2 R\). Use the model to find the probability of finding minerals with a value of at least 93 in a randomly chosen tonne of rock.
CAIE S2 2010 November Q5
5 A continuous random variable \(X\) has probability density function given by $$f ( x ) = \begin{cases} \frac { 1 } { 6 } x & 2 \leqslant x \leqslant 4
0 & \text { otherwise } \end{cases}$$
  1. Find \(\mathrm { E } ( X )\).
  2. Find the median of \(X\).
  3. Two independent values of \(X\) are chosen at random. Find the probability that both these values are greater than 3 .
CAIE S2 2010 November Q6
6 A clinic monitors the amount, \(X\) milligrams per litre, of a certain chemical in the blood stream of patients. For patients who are taking drug \(A\), it has been found that the mean value of \(X\) is 0.336 . A random sample of 100 patients taking a new drug, \(B\), was selected and the values of \(X\) were found. The results are summarised below. $$n = 100 , \quad \Sigma x = 43.5 , \quad \Sigma x ^ { 2 } = 31.56 .$$
  1. Test at the \(1 \%\) significance level whether the mean amount of the chemical in the blood stream of patients taking drug \(B\) is different from that of patients taking drug \(A\).
  2. For the test to be valid, is it necessary to assume a normal distribution for the amount of chemical in the blood stream of patients taking drug \(B\) ? Justify your answer.
CAIE S2 2010 November Q7
7 In the past, the number of house sales completed per week by a building company has been modelled by a random variable which has the distribution \(\mathrm { Po } ( 0.8 )\). Following a publicity campaign, the builders hope that the mean number of sales per week will increase. In order to test at the \(5 \%\) significance level whether this is the case, the total number of sales during the first 3 weeks after the campaign is noted. It is assumed that a Poisson model is still appropriate.
  1. Given that the total number of sales during the 3 weeks is 5 , carry out the test.
  2. During the following 3 weeks the same test is carried out again, using the same significance level. Find the probability of a Type I error.
  3. Explain what is meant by a Type I error in this context.
  4. State what further information would be required in order to find the probability of a Type II error.
CAIE S2 2011 November Q1
1 The random variable \(X\) has the distribution \(\operatorname { Po } ( 1.3 )\). The random variable \(Y\) is defined by \(Y = 2 X\).
  1. Find the mean and variance of \(Y\).
  2. Give a reason why the variable \(Y\) does not have a Poisson distribution.
CAIE S2 2011 November Q2
2 An engineering test consists of 100 multiple-choice questions. Each question has 5 suggested answers, only one of which is correct. Ashok knows nothing about engineering, but he claims that his general knowledge enables him to get more questions correct than just by guessing. Ashok actually gets 27 answers correct. Use a suitable approximating distribution to test at the \(5 \%\) significance level whether his claim is justified.
CAIE S2 2011 November Q3
3 Three coats of paint are sprayed onto a surface. The thicknesses, in millimetres, of the three coats have independent distributions \(\mathrm { N } \left( 0.13,0.02 ^ { 2 } \right) , \mathrm { N } \left( 0.14,0.03 ^ { 2 } \right)\) and \(\mathrm { N } \left( 0.10,0.01 ^ { 2 } \right)\). Find the probability that, at a randomly chosen place on the surface, the total thickness of the three coats of paint is less than 0.30 millimetres.
CAIE S2 2011 November Q4
4 The volumes of juice in bottles of Apricola are normally distributed. In a random sample of 8 bottles, the volumes of juice, in millilitres, were found to be as follows. $$\begin{array} { l l l l l l l l } 332 & 334 & 330 & 328 & 331 & 332 & 329 & 333 \end{array}$$
  1. Find unbiased estimates of the population mean and variance. A random sample of 50 bottles of Apricola gave unbiased estimates of 331 millilitres and 4.20 millilitres \({ } ^ { 2 }\) for the population mean and variance respectively.
  2. Use this sample of size 50 to calculate a \(98 \%\) confidence interval for the population mean.
  3. The manufacturer claims that the mean volume of juice in all bottles is 333 millilitres. State, with a reason, whether your answer to part (ii) supports this claim.
CAIE S2 2011 November Q5
5 The management of a factory thinks that the mean time required to complete a particular task is 22 minutes. The times, in minutes, taken by employees to complete this task have a normal distribution with mean \(\mu\) and standard deviation 3.5. An employee claims that 22 minutes is not long enough for the task. In order to investigate this claim, the times for a random sample of 12 employees are used to test the null hypothesis \(\mu = 22\) against the alternative hypothesis \(\mu > 22\) at the \(5 \%\) significance level.
  1. Show that the null hypothesis is rejected in favour of the alternative hypothesis if \(\bar { x } > 23.7\) (correct to 3 significant figures), where \(\bar { x }\) is the sample mean.
  2. Find the probability of a Type II error given that the actual mean time is 25.8 minutes.
CAIE S2 2011 November Q6
6 Customers arrive at an enquiry desk at a constant average rate of 1 every 5 minutes.
  1. State one condition for the number of customers arriving in a given period to be modelled by a Poisson distribution. Assume now that a Poisson distribution is a suitable model.
  2. Find the probability that exactly 5 customers will arrive during a randomly chosen 30 -minute period.
  3. Find the probability that fewer than 3 customers will arrive during a randomly chosen 12-minute period.
  4. Find an estimate of the probability that fewer than 30 customers will arrive during a randomly chosen 2-hour period.
CAIE S2 2011 November Q7
7 \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{3629cb5c-fb4c-43b1-b43f-ce321411b814-3_385_385_982_246} \captionsetup{labelformat=empty} \caption{Fig. 1}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{3629cb5c-fb4c-43b1-b43f-ce321411b814-3_385_380_982_669} \captionsetup{labelformat=empty} \caption{Fig. 2}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{3629cb5c-fb4c-43b1-b43f-ce321411b814-3_390_378_977_1087} \captionsetup{labelformat=empty} \caption{Fig. 3}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{3629cb5c-fb4c-43b1-b43f-ce321411b814-3_390_391_977_1503} \captionsetup{labelformat=empty} \caption{Fig. 4}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{3629cb5c-fb4c-43b1-b43f-ce321411b814-3_392_391_1475_370} \captionsetup{labelformat=empty} \caption{Fig. 5}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{3629cb5c-fb4c-43b1-b43f-ce321411b814-3_392_387_1475_872} \captionsetup{labelformat=empty} \caption{Fig. 6}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{3629cb5c-fb4c-43b1-b43f-ce321411b814-3_392_389_1475_1375} \captionsetup{labelformat=empty} \caption{Fig. 7}
\end{figure} Each of the random variables \(T , U , V , W , X , Y\) and \(Z\) takes values between 0 and 1 only. Their probability density functions are shown in Figs 1 to 7 respectively.
  1. (a) Which of these variables has the largest median?
    (b) Which of these variables has the largest standard deviation? Explain your answer.
  2. Use Fig. 2 to find \(\mathrm { P } ( U < 0.5 )\).
  3. The probability density function of \(X\) is given by $$f ( x ) = \begin{cases} a x ^ { n } & 0 \leqslant x \leqslant 1
    0 & \text { otherwise } \end{cases}$$ where \(a\) and \(n\) are positive constants.
    (a) Show that \(a = n + 1\).
    (b) Given that \(\mathrm { E } ( X ) = \frac { 5 } { 6 }\), find \(a\) and \(n\).
CAIE S2 2011 November Q7
7 \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{6e90d172-0292-4bb6-9d4a-506669076d4e-3_385_385_982_246} \captionsetup{labelformat=empty} \caption{Fig. 1}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{6e90d172-0292-4bb6-9d4a-506669076d4e-3_385_380_982_669} \captionsetup{labelformat=empty} \caption{Fig. 2}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{6e90d172-0292-4bb6-9d4a-506669076d4e-3_390_378_977_1087} \captionsetup{labelformat=empty} \caption{Fig. 3}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{6e90d172-0292-4bb6-9d4a-506669076d4e-3_390_391_977_1503} \captionsetup{labelformat=empty} \caption{Fig. 4}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{6e90d172-0292-4bb6-9d4a-506669076d4e-3_392_391_1475_370} \captionsetup{labelformat=empty} \caption{Fig. 5}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{6e90d172-0292-4bb6-9d4a-506669076d4e-3_392_387_1475_872} \captionsetup{labelformat=empty} \caption{Fig. 6}
\end{figure} \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{6e90d172-0292-4bb6-9d4a-506669076d4e-3_392_389_1475_1375} \captionsetup{labelformat=empty} \caption{Fig. 7}
\end{figure} Each of the random variables \(T , U , V , W , X , Y\) and \(Z\) takes values between 0 and 1 only. Their probability density functions are shown in Figs 1 to 7 respectively.
  1. (a) Which of these variables has the largest median?
    (b) Which of these variables has the largest standard deviation? Explain your answer.
  2. Use Fig. 2 to find \(\mathrm { P } ( U < 0.5 )\).
  3. The probability density function of \(X\) is given by $$f ( x ) = \begin{cases} a x ^ { n } & 0 \leqslant x \leqslant 1
    0 & \text { otherwise } \end{cases}$$ where \(a\) and \(n\) are positive constants.
    (a) Show that \(a = n + 1\).
    (b) Given that \(\mathrm { E } ( X ) = \frac { 5 } { 6 }\), find \(a\) and \(n\).
CAIE S2 2011 November Q1
1 Test scores, \(X\), have mean 54 and variance 144. The scores are scaled using the formula \(Y = a + b X\), where \(a\) and \(b\) are constants and \(b > 0\). The scaled scores, \(Y\), have mean 50 and variance 100. Find the values of \(a\) and \(b\).
\(235 \%\) of a random sample of \(n\) students walk to college. This result is used to construct an approximate \(98 \%\) confidence interval for the population proportion of students who walk to college. Given that the width of this confidence interval is 0.157 , correct to 3 significant figures, find \(n\).
CAIE S2 2011 November Q3
3 Jack has to choose a random sample of 8 people from the 750 members of a sports club.
  1. Explain fully how he can use random numbers to choose the sample. Jack asks each person in the sample how much they spent last week in the club café. The results, in dollars, were as follows. $$\begin{array} { l l l l l l l l } 15 & 25 & 30 & 8 & 12 & 18 & 27 & 25 \end{array}$$
  2. Find unbiased estimates of the population mean and variance.
  3. Explain briefly what is meant by 'population' in this question.
CAIE S2 2011 November Q4
4 The random variable \(X\) has probability density function given by $$\mathrm { f } ( x ) = \begin{cases} k \mathrm { e } ^ { - x } & 0 \leqslant x \leqslant 1
0 & \text { otherwise } \end{cases}$$
  1. Show that \(k = \frac { \mathrm { e } } { \mathrm { e } - 1 }\).
  2. Find \(\mathrm { E } ( X )\) in terms of e.
CAIE S2 2011 November Q5
2 marks
5 Records show that the distance driven by a bus driver in a week is normally distributed with mean 1150 km and standard deviation 105 km . New driving regulations are introduced and in the next 20 weeks he drives a total of 21800 km .
  1. Stating any assumption(s), test, at the \(1 \%\) significance level, whether his mean weekly driving distance has decreased.
  2. A similar test at the \(1 \%\) significance level was carried out using the data from another 20 weeks. State the probability of a Type I error and describe what is meant by a Type I error in this context.
    [0pt] [2]
CAIE S2 2011 November Q6
6 Ranjit goes to mathematics lectures and physics lectures. The length, in minutes, of a mathematics lecture is modelled by the variable \(X\) with distribution \(\mathrm { N } \left( 36,3.5 ^ { 2 } \right)\). The length, in minutes, of a physics lecture is modelled by the independent variable \(Y\) with distribution \(\mathrm { N } \left( 55,5.2 ^ { 2 } \right)\).
  1. Find the probability that the total length of two mathematics lectures and one physics lecture is less than 140 minutes.
  2. Ranjit calculates how long he will need to spend revising the content of each lecture as follows. Each minute of a mathematics lecture requires 1 minute of revision and each minute of a physics lecture requires \(1 \frac { 1 } { 2 }\) minutes of revision. Find the probability that the total revision time required for one mathematics lecture and one physics lecture is more than 100 minutes.
CAIE S2 2011 November Q7
7 The numbers of men and women who visit a clinic each hour are independent Poisson variables with means 2.4 and 2.8 respectively.
  1. Find the probability that, in a half-hour period,
    (a) 2 or more men and 1 or more women will visit the clinic,
    (b) a total of 3 or more people will visit the clinic.
  2. Find the probability that, in a 10 -hour period, a total of more than 60 people will visit the clinic.
CAIE S2 2012 November Q1
1
\includegraphics[max width=\textwidth, alt={}, center]{879cb813-2380-47a7-bd96-cad0a74d0b4d-2_369_531_255_806} The diagram shows the graph of the probability density function, f , of a random variable \(X\). Find the median of \(X\).
CAIE S2 2012 November Q2
2 The heights of a certain type of plant have a normal distribution. When the plants are grown without fertilizer, the population mean and standard deviation are 24.0 cm and 4.8 cm respectively. A gardener wishes to test, at the \(2 \%\) significance level, whether Hiergro fertilizer will increase the mean height. He treats 150 randomly chosen plants with Hiergro and finds that their mean height is 25.0 cm . Assuming that the standard deviation of the heights of plants treated with Hiergro is still 4.8 cm , carry out the test.
CAIE S2 2012 November Q3
3 The cost of hiring a bicycle consists of a fixed charge of 500 cents together with a charge of 3 cents per minute. The number of minutes for which people hire a bicycle has mean 142 and standard deviation 35.
  1. Find the mean and standard deviation of the amount people pay when hiring a bicycle.
  2. 6 people hire bicycles independently. Find the mean and standard deviation of the total amount paid by all 6 people.
CAIE S2 2012 November Q4
4 A cereal manufacturer claims that \(25 \%\) of cereal packets contain a free gift. Lola suspects that the true proportion is less than \(25 \%\). In order to test the manufacturer's claim at the \(5 \%\) significance level, she checks a random sample of 20 packets.
  1. Find the critical region for the test.
  2. Hence find the probability of a Type I error. Lola finds that 2 packets in her sample contain a free gift.
  3. State, with a reason, the conclusion she should draw.
CAIE S2 2012 November Q5
5 A random variable \(X\) has probability density function given by $$f ( x ) = \begin{cases} \frac { k } { x - 1 } & 3 \leqslant x \leqslant 5
0 & \text { otherwise } \end{cases}$$ where \(k\) is a constant.
  1. Show that \(k = \frac { 1 } { \ln 2 }\).
  2. Find \(a\) such that \(\mathrm { P } ( X < a ) = 0.75\).
CAIE S2 2012 November Q6
6 In order to obtain a random sample of people who live in her town, Jane chooses people at random from the telephone directory for her town.
  1. Give a reason why Jane's method will not give a random sample of people who live in the town. Jane now uses a valid method to choose a random sample of 200 people from her town and finds that 38 live in apartments.
  2. Calculate an approximate \(99 \%\) confidence interval for the proportion of all people in Jane's town who live in apartments.
  3. Jane uses the same sample to give a confidence interval of width 0.1 for this proportion. This interval is an \(x \%\) confidence interval. Find the value of \(x\).
CAIE S2 2012 November Q7
7 A random variable \(X\) has the distribution \(\operatorname { Po } ( 1.6 )\).
  1. The random variable \(R\) is the sum of three independent values of \(X\). Find \(\mathrm { P } ( R < 4 )\).
  2. The random variable \(S\) is the sum of \(n\) independent values of \(X\). It is given that $$\mathrm { P } ( S = 4 ) = \frac { 16 } { 3 } \times \mathrm { P } ( S = 2 )$$ Find \(n\).
  3. The random variable \(T\) is the sum of 40 independent values of \(X\). Find \(\mathrm { P } ( T > 75 )\).