AHSME 1973 Problem 26

The number of terms in an A.P. (Arithmetic Progression) is even. The sum of the odd and even-numbered terms are 24 and 30, respectively. If the last term exceeds the first by 10.5, the number of terms in the A.P. is $\textbf{(A)}\ 20 \textbf{(B)}\ 18 \textbf{(C)}\ 12 \textbf{(D)}\ 10 \textbf{(E)}\ 8$