Interview Q&A / Aptitude

1. A pipe fills a tank in 10 hours, but a leak at the bottom can empty the full tank in 15 hours. With both working, how long does the tank take to fill?

   30 hours. Net rate = 1/10 - 1/15 = (3 - 2)/30 = 1/30 of the tank per hour. With 30 units: the pipe adds 3, the leak removes 2, net 1 unit per hour.

   BeginnerQuick answer1 min read
2. A pipe normally fills a tank in 8 hours, but because of a leak it takes 10 hours. How long would the leak take to empty a full tank?

   40 hours. Leak rate = normal rate - actual rate = 1/8 - 1/10 = 1/40 of the tank per hour. With 40 units: the pipe adds 5, the net is 4, so the leak removes 1 unit per hour.

   BeginnerQuick answer1 min read
3. Pipes A and B fill a tank in 12 and 15 minutes. Pipe C empties it in 20 minutes. If all three are opened together on an empty tank, how long will it take to fill?

   10 minutes. With 60 units of tank: A adds 5, B adds 4, C removes 3 per minute. Net 6 units per minute, so 60/6 = 10 minutes.

   BeginnerQuick answer1 min read
4. Pipes A and B can fill a tank in 20 and 30 minutes. Both are opened, and after 5 minutes B is closed. How much longer does A take to fill the tank?

   11 minutes 40 seconds. With 60 units: together they add 5 units a minute, so 25 units in 5 minutes. A then fills the remaining 35 units at 3 a minute, taking 35/3 = 11 2/3 minutes.

   IntermediateQuick answer1 min read
5. Two pipes fill a cistern in 36 and 45 minutes. A waste pipe can empty it in 30 minutes. If all three are opened together, how long does the cistern take to fill?

   60 minutes. With 180 units: the inlets add 5 and 4 a minute and the waste pipe removes 6, so the net is 3 units a minute and 180/3 = 60 minutes.

   IntermediateQuick answer1 min read