Evan washes two types of vehicles. It takes him 30 minutes to wash a car and 40 minutes to wash a truck. He charges $12 to wash a car and $15 to wash a truck. In 270 minutes, Evan made $105 washing cars and trucks. How many trucks did Evan wash?

Answer 1

30c+40t = 270
12c+15t = 105

now, from the 2nd equation we see that

12c = 105 - 15t, so
30c = 5/2 (105-15t)

plugging that into the 1st equation, we get

5/2(105-15t) + 40t = 270
5(105-15t) + 80t = 540
105-15t+16t=108
t = 3

Answer 2

t=3