A tank can be filled by pipe A in 3 hours and by pipe B in 5 hours. At 7 am pipe A was opened. At what time will the tank be filled if pipe B was opened at 8 am?

The tank fills at 9:15 am.

From 7 to 8 am, A alone fills 1/3, leaving 2/3.

From 8 am both fill at 1/3 + 1/5 = 8/15 per hour.

Time for 2/3 = (2/3)/(8/15) = 5/4 hours = 1 hr 15 min.

So 8:00 + 1:15 = 9:15 am.