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 answers.
A) 12 days
B) 15 days
C) 16 days
D) 18 days
E) 22 days
A's 2 day's work = (1/20 × 2) = 1/10	.   (A + B + C)'s 1 day's work = (1/20 + 1/30 + 1/60) = 1/10.   Work done in 3 days = (1/10 + 1/10) = 1/5.   Now,	1/5	work is done in 3 days.   Therefore, Whole work will be done in (3 x 5) = 15 days.   CORRECT ANSWER: "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?