A, B and C can do a piece of work in 20, 30 and 60 days respectively. In how many days can A do the work if he is assisted by B and C on every third day?  Choose the correct choice from the following options:
A) 12 days B) 15 days C) 16 days D) 18 days E) 20 days
Explanation:   A's 2 day's work =(1/20 x 2)=1/10   (A + B + C)'s 1 day's work =(1/20 +1/30+1/60)=6/60=1/10   Work done in 3 days =(1/10 +1/10)=1/5   Now,	1/5work is done in 3 days   Whole work will be done in (3 x 5) = 15 days.   ANSWER IS 15 days
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A, B and C can do a piece of work in 20, 30 and 60 days respectively. In how many days can A do the work if he is assisted by B and C on every third day?