WBCS.LIVE practice question

Three pipes P, Q and R can separately fill a tank in 10, 15 and 20 hours. P was filled at 7 am, Q at 8 am, and R at 9 am. By what time will the tank be completely filled, if R can work only for a period of 3 hours at a time and after that he has to stop for 1 hour?

Solve this multiple-choice question and check the correct answer with its explanation below.