Pipes A and B can fill a tank in 20 hours and 30 hours, respectively. C is an outlet pipe attached to the tank. If all the three pipes are opened simultaneously, it takes 90 minutes more time than what A and B together take to fill the tank. A and B are kept opened for 7 hours and then closed and C was opened. C will now empty the tank in?

63

Efficiency of A & B = 5 ltr/hr

Time to fill the tank by (A + B) = \(\frac{60}{5}\) = 12 Hour

*(90 minute  = 1.5 Hours)*

ATQ,

Time taken by A, B and C to fill the tank = 12 + 1.5 = 13.5

(A + B - C) × 13.5 = 60

(5 - C) × 13.5 = 60

C = 5 - \(\frac{60}{13.5}\)

C =  \(\frac{5}{9}\)

(A + B) fill the tank in 7 hrs = 5 × 7 = 35 ltr.

then, C will empty it in = \(\frac{35}{\frac{5}{9}}\) = 63 Hours