AHSME 1952 Problem 10

An automobile went up a hill at a speed of $10$ miles an hour and down the same distance at a speed of $20$ miles an hour. The average speed for the round trip was: $\textbf{(A) \ }12\frac{1}{2}\text{mph} \textbf{(B) \ }13\frac{1}{3}\text{mph} \textbf{(C) \ }14\frac{1}{2}\text{mph} \textbf{(D) \ }15\text{mph} \textbf{(E) \ }\text{none of these}$