AMC 12 2007b Problem 2

A college student drove his compact car 120 miles home for the weekend and averaged 30 miles per gallon. On the return trip the student drove his parents' SUV and averaged only 20 miles per gallon. What was the average gas mileage, in miles per gallon, for the round trip?

$\mathrm {(A)} 22 \mathrm {(B)} 24 \mathrm {(C)} 25 \mathrm {(D)} 26 \mathrm {(E)} 28$