Answer
Less than 15 crossings.
$x\lt 15$
Work Step by Step
First, let $x$ be the number of crossings per a three month period.
Then, from the asked problem we have the inequality:
$ 7.50+0.50x\lt\frac{30}{2}$
Next, all we have to do is solve the inequality above.
$ 7.50+0.50x\lt\ 15$
Subtract $7.50$ from both sides.
$ 7.50+0.50x-7.50\lt\ 15 - 7.50$
$ 0.50x\lt\ 7.50$
Divide with $0.50$ (or multiply with $2$) and we get:
$x \lt 15$
Which is our solution.
