A and B can do a piece of work in 12 days, B and C in 15 days, C and A in 20 days. How long would A take separately to do the same work? , A and B can do a piece of work in 12 days, B and C in 15 days, C and A in 20 days. How long would A take separately to do the same work?

Solution

C. 30 days

Explanation

 (A+B)’s one day work = 
(B+C)’s one day work = 
(C+A)’s one day work = 
(A+B+C)’s one day work = 

If A do work alone, then,
A’s one day work = (A+B+C)’s one day work – (B+C)’s one day work
1/10 - 1/15 =(3-2)/30 =1/30 
Thus, A can complete the work in  = 30 days.