Paper 1 · conditional probability
A head of school can contact parents by e-mail, letter or cellphone with probabilities 0.4, 0.1 and 0.5 respectively, using only one method. The probability that parents receive the message is 0.6 if e-mail is used, 0.8 if letter is used and 1 if cellphone is used. What is the probability that the parents receive the message?
Model answer
0.82
Explanation: By the law of total probability, P(receive) = 0.4(0.6) + 0.1(0.8) + 0.5(1) = 0.24 + 0.08 + 0.50 = 0.82.
Derived from ZIMSEC Statistics Paper 1, November 2018, Q2
