Brain Teasers
Prime Suspects
Probability
Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.Probability
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?
Hint
Remember that 1 is not a prime number.Answer
Those 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.
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