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 $\dfrac{98}{25}$. Then the sum of the first $21$ terms of an A.P., whose first term is $10ar$, $n^{\text{th}}$ term is $a_n$ and the common difference is $10ar^{2}$, is equal to: