AHSME 1950 Problem 19

If $m$ men can do a job in $d$ days, then $m+r$ men can do the job in: $\textbf{(A)}\ d+r \text{ days} \textbf{(B)}\ d-r\text{ days} \textbf{(C)}\ \frac{md}{m+r}\text{ days} \\ \textbf{(D)}\ \frac{d}{m+r}\text{ days} \textbf{(E)}\ \text{None of these}$