Pipes A, B and C can fill a tank in 20, 30 and 60 hours, respectively. Pipes A, B and C are opened at 7 am, 8 am and 9 am respectively. When will the tank full?

5:40PM

⇒ 7am to 8am : Tank fill by A = 3 ltr

⇒ 8am to 9am : Tank fill by A + B = 3 + 2 = 5 ltr

Tank filled till 9 AM = 8 ltr

Remaining tank = 60 - 8 = 52

Tank will fill by all three pipes after → 

⇒ \(\frac{52}{3+2+5}\) = 8\(\frac{2}{3}\) hrs.

or 9 + 8\(\frac{2}{3}\) = 5:40 PM