Explanation

(b)
Given series is 148, 146, 144, 142,... whose first term and common difference is
$a = 148,\ d = (146 - 148) = - 2$
$S_{n} = \frac{n}{2}\left\lbrack 2a + (n + 1)d \right\rbrack = 125$ (given)
$\Rightarrow \ 125n = \frac{n}{2}\lbrack 2 \times 148 + (n - 1) \times ( - 2)\rbrack$
$\Rightarrow \ n^{2} - 24n = 0\ \ \Rightarrow \ \ n(n - 24) = 0$
$\Rightarrow \ \ n = 24\ \ \ (n \neq 0)$