If A can do a piece of work in 15 days and B can do it in 20 days, how long will it take for both A and B to complete the work together?

Answer: B. 10 days
The combined work rate of A and B is 1/15 + 1/20 = 7/60. Therefore, they will complete the work in 60/7 = approximately 8.57 days.
Posted in:  Time & Work -  Aptitude -  Mathematics