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 ?

A’s 2 day’s work = 1/20 x 2
= 1/10
(A + B + C) 1 day’s Work = (1/20 + 1/30 + 1/60)
= 6/60
= 1/10
Now,
Work done in 3 days = (1/10) + (1/10)
= 1/5
Now 1/5 work is done in 3 days.
Now whole work will be done in 3 * 5 = 15 days.