Ans: $48$
Let $x$ and $y$ be the number of stickers owned by the boy and the girl respectively.
Let $x$ and $y$ be the number of stickers owned by the boy and the girl respectively.
$\left\{\begin{array}{ll}
x = 3y & \ldots \unicode{x2460} \\
2(x-20) = y+20 & \ldots \unicode{x2461}
\end{array}\right.$
Sub. $\unicode{x2460}$ into $\unicode{x2461}$, we have
$\begin{array}{rcl}
2(3y-20)& = & y +20 \\
6y – 40 & = & y + 20 \\
5y & = & 60 \\
y & = & 12
\end{array}$
Sub. $y=12$ into $\unicode{x2460}$, we have
$\begin{array}{rcl}
x & = & 3(12) \\
x & = 36
\end{array}$
$\therefore$ the boy owned $36$ stickers and the girl owned $12$ stickers.
Hence, the total number of stickers
$\begin{array}{cl}
= & 36 + 12 \\
= & 48
\end{array}$
