Question

Two pipes A and B can separately fill a cistern in 15 hours and 20 hours respectively. There is a third
pipe at the bottom to empty it. If all the three pipes are simultaneously opened, then the cistem is filled
in 30 hours. In how much time the third pipe alone can empty the cistern?​

Answer 1

Answer:

Part of the tank filled when all the three pipes are opened for 10 hours

=10×(1 /15 + 1 /20 −1 / 25 )=10×

(20+15−12 / 300 )= 230 /300 = 23 / 30

Remaining empty part = 1− 23 / 30 = 7 / 30

Part of the tank filled in 1 hour by pipes (A and B) = 1 /15 +1 /20 = 4+3 / 60 = 7 / 60

∵ 7 /60 part is filled in 1 hour 7/ 30

part is filled in 2 hours

∴ Total time to fill the tank =10+2=12 hours.

Hope this is helpful for you.