Ans: $40$
Let $x$ and $y$ be the number of apples owned by Ada and Billy respectively.
Let $x$ and $y$ be the number of apples owned by Ada and Billy respectively.
$\left\{ \begin{array}{ll}
x = 4y & \ldots \unicode{x2460} \\
x-12 = y + 12 & \ldots \unicode{x2461}
\end{array}\right.$
Sub. $\unicode{x2460}$ into $\unicode{x2461}$, we have
$\begin{array}{rcl}
4y – 12 & = & y + 12 \\
3y & = & 24 \\
y & = & 8
\end{array}$
Sub. $y=8$ into $\unicode{x2460}$, we have
$\begin{array}{rcl}
x & = & 4(8) \\
x & = & 32
\end{array}$
Therefore, Ada has $32$ apples and Billy has $8$ apples.
Hence, the total number of apples they have
$\begin{array}{cl}
= & 32 + 8 \\
= & 40
\end{array}$
