P, Q and R can do a work in 20, 30 and 60 days respectively. How many days does it need to complete the work if P does the work and he is assisted by Q and R on every third day?  Choose the correct choice from the following answers
A) 10  B) 5  C) 15  D) 20  E) 25
Amount of work P can do in 1 day = 1/20   Amount of work Q can do in 1 day = 1/30   Amount of work R can do in 1 day = 1/60   P is working alone and every third day Q and R is helping him   Work completed in every three days = 2 × (1/20) + (1/20 + 1/30 + 1/60) = 1/5   So work completed in 15 days = 5 × 1/5 = 1   Ie, the work will be done in 15 days.   Answer is 15.
<result>C</result>
P, Q and R can do a work in 20, 30 and 60 days respectively. How many days does it need to complete the work if P does the work and he is assisted by Q and R on every third day?