CAIE S1 2013 November — Question 7

Exam BoardCAIE
ModuleS1 (Statistics 1)
Year2013
SessionNovember
TopicProbability Definitions
TypeSequential events and tree diagrams

7 James has a fair coin and a fair tetrahedral die with four faces numbered 1, 2, 3, 4. He tosses the coin once and the die twice. The random variable \(X\) is defined as follows.
  • If the coin shows a head then \(X\) is the sum of the scores on the two throws of the die.
  • If the coin shows a tail then \(X\) is the score on the first throw of the die only.
    1. Explain why \(X = 1\) can only be obtained by throwing a tail, and show that \(\mathrm { P } ( X = 1 ) = \frac { 1 } { 8 }\).
    2. Show that \(\mathrm { P } ( X = 3 ) = \frac { 3 } { 16 }\).
    3. Copy and complete the probability distribution table for \(X\).
\(x\)12345678
\(\mathrm { P } ( X = x )\)\(\frac { 1 } { 8 }\)\(\frac { 3 } { 16 }\)\(\frac { 1 } { 8 }\)\(\frac { 1 } { 16 }\)\(\frac { 1 } { 32 }\)
Event \(Q\) is 'James throws a tail'. Event \(R\) is 'the value of \(X\) is 7'.
  • Determine whether events \(Q\) and \(R\) are exclusive. Justify your answer.