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:
A) 20 B) 18 C) 12 D) 14 E) 15
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 = (3+2+1)/60 = 1/10   Work done in 3 days = A's 2 day's work + (A + B + C)'s 1 day's work   Work done in 3 days = 1/10 + 1/10   Now, work is done in 3 days = 2/10 = 1/5   So, 1/5 cork is done in 3 days.   So, work will be done in (3 x 5) = 15 days.   So, correct answer is 15
<result>E</result>
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?