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: