Let's note:
x=the number of crossing per three-month period

We write the inequality which models the problem: \(\displaystyle{7.50}+{0.50}{x}{<}\frac{{30}}{{2}}\)</span>

We solve the inequality: 7.50+0.50x

7.50+0.50x-7.50

0.50x

\(\displaystyle{x}{<}\frac{{7.50}}{{0.50}}\)</span>

x<15

We write the inequality which models the problem: \(\displaystyle{7.50}+{0.50}{x}{<}\frac{{30}}{{2}}\)</span>

We solve the inequality: 7.50+0.50x

7.50+0.50x-7.50

0.50x

\(\displaystyle{x}{<}\frac{{7.50}}{{0.50}}\)</span>

x<15