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?
Answer: B
LCM of 20, 30, 60 is 60. Total work = 60 units. A's rate=3, B's rate=2, C's rate=1 unit/day. A works alone for the first two days, doing \(3+3=6\) units. On the third day, A, B, and C work together, doing \(3+2+1=6\) units. So, in a 3-day cycle, \(6+6=12\) units of work are done. To complete 60 units, we need \(60/12 = 5\) such cycles. The total time taken is \(5 \times 3 = 15\) days.