Jack picks at random three integers between 7 and 11 (inclusive). Mary picks an integer from 24 to 33 (inclusive). What is the probability that Mary’s number is less than the sum of Jack’s numbers ?

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Serge Hazanov


Here is my solution:

The expected value of Jacks’ number is 9, so the expected value of the sum is 27.

The required probability is 3/10 = 0.3