1+2+3+…(n-1)=n(n-1)/2

Each unordered pair of green balls corresponds uniquely with a blue one. So the set of pairs of green balls has the same order as the set of blue balls. Since \tbinom{5}{2} pairs are possible and the blue balls are 1+2+3+4 in number so 1+2+3+4 = \tbinom{5}{2}=5(5-1)/2

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