Ans: $\$6$
Let $\$x$ and $\$y$ be the cost of a bottle of orange juice and a bottle of milk respectively. Then, we have
Let $\$x$ and $\$y$ be the cost of a bottle of orange juice and a bottle of milk respectively. Then, we have
$\left\{ \begin{array}{ll}
x=2y & \ldots \unicode{x2460} \\
3x+5y = 66 & \ldots \unicode{x2461}
\end{array} \right. $
Sub. $\unicode{x2460}$ into $\unicode{x2461}$, we have
$\begin{array}{rcl}
3(2y)+5y & = & 66 \\
11y & = & 66 \\
y & = & 6
\end{array}$
Therefore, the cost of a bottle of milk is $\$6$.
