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 dollars to wash a car and 15 dollars to wash a truck. In 270 minutes, Evan made 105 dollars washing cars and trucks. How many trucks did Evan wash?

Let c be the number of cars and t the number of trucks. The time equation is 30c + 40t = 270, and the earnings equation is 12c + 15t = 105. Multiplying the time equation by 0.4 gives 12c + 16t = 108; subtracting the earnings equation leaves t = 3. Evan washed 3 trucks, which is choice C.

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