Posted by indranil chatterjee on Monday, May 2, 2011 at 12:56pm.
Say the probability he'll pass English is Prob(E) = 0.6. The probability he'll pass in both English and Maths is Prob(E&M) = 0.54. Provided the probability that he'll pass English is independent of the probability that he'll pass Maths (and note that's an assumption we're making that may or may not be true), then the probability that he'll pass Maths must be Prob(M) = Prob(E&M) / Prob(E) = 0.54/0.6 = 0.9, because Prob(E&M) = Prob(E) x Prob(M) if the events are independent. So Prob(M) = 0.9. But we were asked for the probability that he'll fail Maths, not pass it, and that's 1 - Prob(M). So the answer must be 1 - 0.9 = 0.1. Do you agree?
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