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?
Amount of work P can do in 1 day =120
Amount of work Q can do in 1 day =130
Amount of work R can do in 1 day =160
P is working alone and every third day Q and R is helping him
Work completed in every three days =2×(120)+(120+130+160)=15
So work completed in 15 days =5×15=1, the work will be done in 15 days.