Answer
20 times
Work Step by Step
Consider the equation for the condition at which the toll for the bridge with discount pass is equal to the toll for the bridge without discount
$5x=30+3.50x$
Here, x is the number of times in a month the bridge must be used
Now, solve the above equation
$\begin{align}
& 5x=30+3.50x \\
& 5x-3.50x=30 \\
& 1.5x=30 \\
& x=20
\end{align}$
Thus, the number of times in a month the bridge must be used is 20.
