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counting and probability senior
Problem
Krishanu and Shaunak each pick an integer at random between 1 and 10, inclusive. What is the probability that the product of their numbers is more than 10? Express your answer as a common fraction.
Solution
Since 10 is small for a product, we consider the complementary probability, that their product is at most 10. To do this, we count the number of ordered pairs of positive integers with ( is Krishanu's number and is Shaunak's number). If , there are 10 such pairs; if , there are 5; if there are 3; if or , there are 2, and if there is 1, for a total of ordered pairs with product at most 10. The probability one of these is picked is then , since there are possible ordered pairs. Therefore, the probability that the product of their numbers is greater than 10 is .
Final answer
\frac{73}{100}