If the sum of an infinite GP \( a, ar, ar^{2}, ar^{3}, \ldots \) is \( 15 \) and the sum of the squares of its each term is \( 150 \), then the sum of \( ar^{2}, ar^{4}, ar^{6}, \ldots \) is:
\( \dfrac{1}{2} \)
\( \dfrac{9}{2} \)
\( \dfrac{5}{2} \)
\( \dfrac{25}{2} \)