SSC CHSL 12 Apr 2021 Shift 2 Paper

Let the number of boys and girls be $\mathit{x}$ and $\mathit{y}$, respectively.
$\therefore$ According to the question,
$5\mathit{x}+7\mathit{y}=85$ . . . (i)
$4\mathit{x}+9\mathit{y}=85$ . . . (ii)
From Eqs. (i) and (ii),
$5\mathit{x}+7\mathit{y}=4\mathit{x}+9\mathit{y}$
$\mathit{x}=2\mathit{y}$
Put the value of $\mathit{x}$ in Eq. (i),
$5\times 2\mathit{y}+7\mathit{y}=85$
$\Rightarrow 17\mathit{y}=85$
$\Rightarrow \mathit{y}=\frac{85}{17}=5$
$\therefore$ Total number of students $=\mathit{x}+\mathit{y}$
$=2\mathit{y}+\mathit{y}=3\mathit{y}$
$=3\times 5=15$