Question

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?

Answer 1

Amount of work P can do in 1 day $=\frac{1}{20}$
Amount of work Q can do in 1 day $=\frac{1}{30}$
Amount of work R can do in 1 day $=\frac{1}{60}$
P is working alone and every third day Q and R is helping him
Work completed in every three days $=2\times \left(\frac{1}{20}\right)+(\frac{1}{20}+\frac{1}{30}+\frac{1}{60})=\frac{1}{5}$
So work completed in 15 days $=5\times \frac{1}{5}=1$, the work will be done in 15 days.