It is part of the series known as the triangular numbers.
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21: (3)+3=(6)+4=(10)+5=(15)+6=(21)
5/7 - 5/21 = 15/21 - 5/21 = 10/215/7 - 5/21 = 15/21 - 5/21 = 10/215/7 - 5/21 = 15/21 - 5/21 = 10/215/7 - 5/21 = 15/21 - 5/21 = 10/21
The difference increases by one. 3, 6, 10, 15, 21, 28, 36 and so on.
7/10 of 15 = 15*7/10 = 21/2
25 + 15 + 21 + 10 + 11 = 82