Three pipes A, B and C working together can fill a tank in 6 hours. If A can fill it in 15 hours and B can fills it in 20 hours, then how much time will C take to fill the tank alone?

10 hrs 20 hrs 40 hrs 15 hrs

20 hrs

Efficiency of C = Eff. of (A + B + C) - Eff. of ( A + B )                        = 10 - 7 = 3  C will fill it in → \(\frac{60}{3}\) = 20 hours.