Hi Anna;
x=quantity of tshirts
$15+$30x=$135
Let's subtract $15 from both sides...
$15+$15+$30x=$135$15
$30x=$120
Let's divide both sides by $30...
($30x)/$30=$120/$30
Note that the units of $ are in each numerator and denominator. Henceforth, these cancel.
(30x)/30=120/30
x=4
1/6/2014

Vivian L.