Get premium membership and access revision papers with marking schemes, video lessons and live classes.
Two pipes P and Q each running alone can fill a trough in 6 hours and 10 hours respectively. A drainage pipe R can empty the full trough in 15 hours. Pipes P and Q are turned on and left running for 121 hours. The drainage pipe R is then opened and all three-left running. Find how
much longer it takes to fill the trough.