#### Answer

The required value is $\$12\text{ per hour}$.

#### Work Step by Step

Let us assume $x$ is the normal salary per hour.
After $40$ hours, the salary per hour is:
$\begin{align}
& =x+\frac{x}{2} \\
& =\frac{3x}{2} \\
\end{align}$
We have earned $\$660$ for $50$ hours of work.
So, salary until 40 hours is $=40x$
After $40$ hours, for the remaining $10$ hours the salary is
$\begin{align}
& =10\times \frac{3x}{2} \\
& =15x
\end{align}$
Thus, the total salary is:
$\begin{align}
& 40x+15x=660 \\
& 55x=660 \\
& x=12
\end{align}$