Explanation

(c)
Let the number of girls in the class $= y$
$\therefore$ Number of boys in the class $= 100 - y$
Now, ${\overline{x}}_{1} = 25,\ n_{1} = y,\ {\overline{x}}_{2} = 50,\ n_{2} = 100 - y$
and $\overline{x} = 35,\ n_{1} + n_{2} = 100$
$\therefore\ \ \ 35 = \frac{25 \times y + 50 \times (100 - y)}{100}$
$\Rightarrow \ \ 3500 = 25y + 5000 - 50y$
$\Rightarrow \ \ 25y = 1500\ \ \Rightarrow \ \ y = 60$
$\therefore$ Number of girls in the class $= 60$