Maths Olympiad Prep

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Problem 728

AMC 10/12, early questions
Combinatorics Difficulty 4.0 Find the answer United States competition problems · United States

How many distinct sets of 8 positive odd integers sum to 20?

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

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Official solution

Solution:

This is the same as the number of ways 8 nonnegative even integers sum to 12 (we subtract 1 from each integer in the above sum). All 11 possibilities are (leaving out 0s): 1212, 10+210+2, 8+48+4, 8+2+28+2+2, 6+66+6, 6+4+26+4+2, 6+2+2+2+26+2+2+2+2, 4+4+44+4+4, 4+4+2+24+4+2+2, 4+2+2+2+24+2+2+2+2, 2+2+2+2+2+22+2+2+2+2+2.

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