Three taps A, B and C can fill a tank in 12 hrs, 15 hrs and 20 hrs respectively. Tap A opened at 7 am, Tap B opened at 10 am and Tap C opened at 11 am. At what time the tank will be filled completely?

2 pm

⇒ 7 am to 10 am : tank fill by pipe A = 5 × 3 = 15 ltr

⇒ 10 am to 11 am : tank fill by pipe A + B = 9 × 1 = 9 ltr

Remaining tank = 60 - 15 - 9 = 36 ltr

Time take to fill the tank by A + B + C after 11 am = \(\frac{36}{12}\) = 3 hrs.

Hence, the tank will be filled at = 11am + 3 hrs. = 2 pm.