Answer
The costs are equal when the bridge is crossed 20 times.
Work Step by Step
Let x = number of times the bridge is crossed in a month.
Cost of tolls, no discount = $5x $
Cost using a discount pass = $30+3.5x $
We want the x for which these two costs are equal.
$5x $=$30+3.5x \qquad $ ... add $-3.5x $
$1.5x=30 \qquad $ ... divide with $1.5$
$ x=20.$
The costs are equal when the bridge is crossed 20 times.
