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?  Choices
A) 10 days
B) 14 days
C) 15 days
D) 9 days
E) 5 days
Explanation :   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 :15 days
<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?