AMC 8 2009 Problem 14

Austin and Temple are $50$ miles apart along Interstate 35. Bonnie drove from Austin to her daughter's house in Temple, averaging $60$ miles per hour. Leaving the car with her daughter, Bonnie rode a bus back to Austin along the same route and averaged $40$ miles per hour on the return trip. What was the average speed for the round trip, in miles per hour? $\textbf{(A)}\ 46 \textbf{(B)}\ 48 \textbf{(C)}\ 50 \textbf{(D)}\ 52 \textbf{(E)}\ 54$