Maths Olympiad Prep

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Combinatorics Difficulty 4.9 AIME Find the answer

Start by writing the integers 1,2,4,61,2,4,6 on the blackboard. At each step, write the smallest positive integer nn that satisfies both of the following properties on the board. - nn is larger than any integer on the board currently. - nn cannot be written as the sum of 2 distinct integers on the board. Find the 100-th integer that you write on the board. Recall that at the beginning, there are already 4 integers on the board.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

The sequence goes 1,2,4,6,9,12,17,20,25,1,2,4,6,9,12,17,20,25, \ldots. Common differences are 5,3,5,3,5,3,5,3,5,3,5,3, \ldots, starting from 12. Therefore, the answer is 12+47×8=38812+47 \times 8=388.

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