Ans: $64$
Let $x$ and $y$ be the number of stamps owned by John and Mary respectively.
Let $x$ and $y$ be the number of stamps owned by John and Mary respectively.
$\left\{ \begin{array}{ll}
x+y = 300 & \ldots \unicode{x2460} \\
4x=y+20 & \ldots \unicode{x2461}
\end{array} \right.$
From $\unicode{x2461}$,
$\begin{array}{rcl}
4x & = & y + 20 \\
y & = & 4x – 20~\ldots \unicode{x2462}
\end{array}$
Sub. $\unicode{x2462}$ into $\unicode{x2460}$, we have
$\begin{array}{rcl}
x + (4x – 20) & = & 300 \\
5x -20 & = & 300 \\
5x & = & 320 \\
x & = & 64
\end{array}$
Therefore, John owns 64 stamps.
