Let the sum of an infinite G.P., whose first term is a and the common ratio is $r$, be 5 . Let the sum of its first five terms be $98/25$. Then the sum of the first 21 terms of an A.P., whose first term is $10ar,n^{\text{th}\text{~}}$ term is $a_{n}$ and the common difference is $10ar^{2}$, is equal to