Two pipes A and B can fill a tank in 4 hours and 7 hours

Solution

A can Fill a tank in 4 hours and B can do the same in 7 hours & C can empty tank in 8 hrs. Let Total Work = LCM (4, 7 & 8) = 56 units Work done in 1st hr by A = 14 units Work done in 2nd hr by A & B = 14+8 = 22 units; Total work done in 2 hrs = 22 + 14 = 36 units, Now pending work = 56 – 36 = 20 units Work done in 3rd hr = 14 + 8 – 7 = 15 units Remaining work = 20 - 15 = 5 units Time taken to complete the remaining work = 5/15 = 1/3 = 20 min Hence work will be completed at = 7am + 3.20 = 10.20 am.