Can you show us any partial attempts you have tried on this problem? (Hint: It will involve derivatives).
A and B can fill a cistern in 7.5 minutes and 5 minutes respectively and C can carry off 14 litres per minute. If the cistern is already full and all the three pipes are opened, then it is emptied in 1 hr. How may liters can it hold?
I took A and B as one pipe which fills 1/3 of the cistern every minute and you have a pipe C emptying 14 liters per minute. The cistern is already full in the begining.
So I took the computation as 1+(1/3)(x)-14 litres=0
so x=3.5 and the total cistern can have 10.5 liters.
But the book answer is 40. Can you help?