A, B and C can do a piece of work in 20, 30 and 60 days resp. In how many days can A do the work if he is assisted by B and C together on every third day?

15

A is assisted by B and C together on every third day

Total work = Efficiency × Number of days

A's 2 day work = 2 × 3R = 6R

on 3rd day B & C joins A , total efficiency on 3rd day = 6R

Total 3 days work = 6R + 6R = 12R

 time taken = \(\frac{60R}{12R}\) = 5

Total time = 5 × 3 = 15 days