A, B and C can do a piece of work in 20, 30 and 60 days respectively. How many days does it needed to complete the work if A does the work and he is assisted by B and C on every third day?

A completes 1/20 of the work daily. B completes 1/30 daily, and C completes 1/60 daily. On every third day, all three work together, completing 1/20 + 1/30 + 1/60 = 6/60 = 1/10 of the work. The 3-day cycle completes 1/20 + 1/20 + 1/10 = 1/5 of the work. In 15 days (5 cycles), they complete 5 × (1/5) = 1, meaning the entire work is finished. The other options don't align with this cyclical pattern.