Two inlet pipes, P1 and P2 can fill a cistern in 20 hours and 30 hours, respectively. They were opened at the same time, but pipe P1 had to be closed 5 hours before the cistern was full. How many hours in total did it take for the two pipes to fill the cistern?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 15 hours.
Pipe P1 fills 1/20 of the cistern per hour.
Pipe P2 fills 1/30 of the cistern per hour.
Let the total time required to fill the cistern = x hours
So, + = 1
⇒ = 1
⇒ 3x - 15 + 2x = 60
⇒ x = = 15