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, 2018

Combinatorics Difficulty 3.0 AMC 10/12 Prove it United Kingdom

In the expression below, three of the $+$ signs are changed into $-$ signs so that the expression is equal to $100$ : $0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20.$ In how many ways can this be done?

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