Answer
The number of bridge crossings per month is $15$. The monthly cost is $\$90$.
Work Step by Step
Let $x$ represent the number of bridge crossings per month.
The monthly cost to avail the discount pass is $\$30$.
The toll for the people with the discount is $\$4$.
The monthly cost without the discount pass is $\$6$ times the number of bridge crossings per month and it is equal to the monthly cost with the discount pass that is $\$4$ times the number of bridge crossings per month plus the monthly cost of the discount pass, which is $\$30$.
Rewrite it as an equation:
$6x=4x+30$
Simplify the equation as shown below to get the value of x,
$\begin{align}
& 6x-4x=30 \\
& 2x=30 \\
& x=\frac{30}{2} \\
& x=15
\end{align}$
The monthly cost without the discount is computed as shown below:
$\begin{align}
& 6x=6\cdot 15 \\
& =90
\end{align}$
The monthly cost with the discount is computed as shown below:
$\begin{align}
& 4x+30=4\cdot 15+30 \\
& =60+30 \\
& =90
\end{align}$
