Question

Two pipes A and B can fill a cistern in 15 hours and 10 hours respectively. A tap C can empty the full cistern in 30 hours. All the three taps were open for 3 hours, when it was remembered that the emptying tap had been left open. It was then closed. How many hours more would it take for the cistern to be filled?
A. 12 hours
B. 15 hours
C. 18 hours
D. 20 hours

Answer 1

(I) Half of the tank is filled in 5 hours
(II) Let the time taken to fill the tank be x hours
Use, `(x)/(15)+(x)/(30)-((x-5))/(20)=1` and find x