Paper 4 · probability - conditional probability

Using the same school (55% own O-level pupils, 45% from elsewhere; pass rates 70% and 90% respectively, overall pass probability 0.79), given that a graduate passed A-level, what is the probability that they did O-level outside the school?

A0.45

B0.487

C0.90

D0.513

Explanation: By Bayes' theorem,

P(\text{outside}|\text{pass}) = P(\text{pass}|\text{outside})P(\text{outside})/P(\text{pass}) = (0.90\times0.45)/0.79 = 0.405/0.79 \approx 0.513

Derived from ZIMSEC Maths Paper 1, November 2008, Q2 (Statistics)

Get the full paper, not just one question

Danho has every sitting for this paper, with your progress tracked question by question, offline.