A can do a work in 15 days and B in 20 days. If they work on it together, then the number of days the whole work can be finished is:
Choose one.
A) 7/40
B) 7/60
C) 40/7
D) 60/7
E) 50/7
A's 1 day's work =1/15   B's 1 day's work =1/20   (A + B)'s 1 day's work =(1/15+1/20)=7/60   the whole work can be finished in 60/7days. Answer: 60/7
<result>D</result>
A can do a work in 15 days and B in 20 days. If they work on it together, then the number of days the whole work can be finished is: