Prime SuspectsProbability puzzles require you to weigh all the possibilities and pick the most likely outcome.
Two hundred people in an auditorium are asked to think of a single digit number from 1 to 9 inclusive and write it down. All those who wrote down a prime number are now asked to leave. Ninety people remain behind in the hall. How many of these are expected to have written down an odd number?
AnswerThose that remain behind must have written {1,4,6,8,9} and from this only {1,9} are odd. The probability of an odd number is thus 2/5.Expected number of odds is 2/5 * 90 = 36. Hide What Next?
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