P, Q and R can complete a work in 30 days, 15 days and 20 days respectively. P works on first day, then Q works on second day and then R works on third day and so on. In how many days the work will be completed?

Let the total work be 60 units (LCM of 30,15 and 20)
Efficiency of P = 60/30 = 2 units per day
Efficiency of Q = 60/15 = 4 units per day
Efficiency of R = 60/20 = 3 units per day
Then, 9 units of work will be completed in 3 days.
=> 54 units of work will be completed in 18 days.
Next 2 units of work will be completed by P in 1 day.
Remaining 4 units of work will be completed by Q in 1 day.
Therefore, Time taken to complete 60 units of work = 18+1+1 = 20 days.