Problem: If m men can do a job in d days, then m+r men can do the job in:
Answer Choices:
A. d+r days
B. d−r days
C. m+rmd​ days
D. m+rd​ days
E. none of the above
Solution:
The job requires md man-days, so that one man can do the job in md days. Therefore, (m+r) men can do the job in md/(m+r) days;
or
dx​=m+rm​;∴x=m+rmd​