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2014 AMC 10B Problem 18

Problem 18 of 25IntermediateAlgebra

A list of 1111 positive integers has a mean of 10,10, a median of 9,9, and a unique mode of 8.8. What is the largest possible value of an integer in the list?

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Solution

The list has total sum 1110=11011\cdot10=110. To maximize the largest entry, minimize the sum of the other ten entries. In nondecreasing order, the sixth entry is 99, and 88 must be the unique mode. If 88 appears twice, the least possible first ten entries sum to 1+2+3+8+8+91+2+3+8+8+9 +10+11+12+13+10+11+12+13 =77=77, giving largest entry 3333. If 88 appears three times, the least possible first ten entries are 1,1,8,8,8,9,9,10,10,111,1,8,8,8,9,9,10,10,11, with sum 7575, giving largest entry 3535. If 88 appears four or five times, the least possible sum of the first ten entries is at least 8080, so the largest entry is at most 3030. Therefore the largest possible entry is 3535, and the correct answer is E .

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Concepts: mean · median (data) · mode · optimization

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.