**Concept:** **The question tests on the area of Rates(Time and Work)**

**Solution:**

LCM of 20,30 and 60 = 60

Assume that 60 units has to be done

Work done by A in 1 day = 60/20 = 3 units

Work done by B in 1 day = 60/30 = 2 units

Work done by C in 1 day = 60/60 = 1 units

A works alone for 2 days and then is assisted by B and C on the 3 rd day.

Work done every 3 days = 3 + 3 + (3 + 2 + 1) = 12 units

To do 60 units, we need 3 * 60/12 = 15 days

Option B