>>13283383

4 10 20.

Diff between 4 & 10 is 6.

Diff between 10 & 20 is 10.

Next # is 35 because 15 is the next difference (each diff is the sum of x as x goes from 1 to n - 1+2+3+4+5=15. Then +6=21. Then +7=28.

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So 35 + 21 =56.

+28=84

+36=120

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