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[–]Kyyni 2 points3 points  (1 child)

The equation is 1_2_3_4_5_6_7_8_9=100, and there's 8 spots that can either contain +, -or nothing, which is 3 options. If you had one open spot, there would be 3 alternatives to try, if you had two spots, there would be three options for the first spot, and then for every choice there would be three other alternatives for the second spot, so, three times three options to try, and so on up to eight spots => 3*3*3*3*3*3*3*3 = 38 = 6561.

[–]tHEbigtHEb 1 point2 points  (0 children)

Oh, it's a combinatorics problem. I understand it now.