Maths Olympiad Prep

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Algebra Difficulty 4.9 AIME Prove it South Africa

What is the minimum number of integers which must be removed from the first 50 positive even integers so that the sum of the remaining integers is 20162016?

Solution

The first 5050 even positive integers are 2,4,6,,96,98,1002, 4, 6, \ldots, 96, 98, 100, and their sum is 50×12(2+100)=50×51=255050 \times \frac{1}{2}(2 + 100) = 50 \times 51 = 2550. To reduce the sum to 20162016 we must subtract 534534. The average of the numbers removed must be less than 100100, so we need to remove at least six. To use as few numbers as possible, we first remove the five largest numbers from 100100 down. Now 100+98+96+94+92=480100 + 98 + 96 + 94 + 92 = 480, which is nearly there, so to bring the total subtracted to 534534 we finally need to remove 5454.

[There are, of course, many other combinations of five integers with a sum of 534534 that can be removed. ]

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