Pipes and Cistern

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Pipes and Cisterns


What is Inlet?

A pipe connected with a tank or a cistern or a reservoir, that fills it, is known as inlet.

What is Outlet?

A pipe connected with a tank or a cistern or a reservoir, emptying it, is known as an outlet.

  • If a pipe can fill a tank in x hours, then:

part filled in 1 hour =

  • If a pipe can empty a full tank in y hours, then:

part emptied in 1 hour =

  • If a pipe can fill a tank in x hours and another pipe can empty the full tank in y hours (where y>x), then on opening both the pipes, the net part filled in 1 hour =
  • If a pipe can fill a tank in x hours and another pipe can empty the full tank in y hours (where x>y), then on opening both the pipes, the net part emptied in 1 hour =

Ques1. A tap can fill a tank in 6 hours. Once the half the tank is filled, three more similar taps are opened. In how much time the tank will fill completely?

  • 3 hrs 30 min ) 3 hrs 45 min                     c.) 4 hrs                    d.) 4 hrs 15 min                      e.) None of these

Explanation: Answer is B

Time taken by one tap to fill half tank = 3 hrs

Some part filled by the four taps in 1 hour =  =

Remaining Part =   =

∴  :  : : 1 : x or x =  =  hrs i.e. 45 mins.

So, total time taken = 3hrs 45 mins.

Ques2. Two pipes X and Y can fill a tank in 20 and 30 minutes respectively. If we use both the pipes together, then how long will it take to fill the tank?


  • 12 min ) 20 min                     c.) 25 min                  d.) 18 min                     e.) None of these

Explanation: Answer is A

Part filled by X in 1 min = ; Part filled by Y in 1 min =

Part filled by (X + Y) in 1 min. =  =

∴ both pipes fill the tank in 12 minutes.

Ques3. A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. If we open both the taps simultaneously then after how much time will the cistern get filled?

  • 5 hrs b.) 5 hrs                 c.) 6.5 hrs                   d.) 7.2 hrs                    e.) None of these

Explanation: Answer is D

Net part filled in 1 hour =  =

∴ cistern will be filled in  hrs i.e. 7.2 hrs

Ques4. A water tank is two-fifth full. Pipe X can fill a tank in 10 minutes and pipe Y can empty it in 6 minutes. If we open both the pipes, then how long will it take to empty or fill the tank completely?

  • 6 min to empty ) 7 min to fill                      c.) 7 min to empty                    d.) 6 min to fill                   e.) None of these

Explanation: Answer is A

Pipe Y is faster than pipe X and so, the tank will be emptied

Part to be emptied =

Part emptied by (X + Y) in 1 minute =  =

∴  :  : : 1 : x or x =  = 6 min

So, the tank will get emptied in 6 min.

Ques5. Pipe X can fill a tank in 5 hours, pipe Y in 10 hours and pipe Z in 30 hours. In how many hours will the tank be filled if all the pipes are open?

  • 2 ) 2.5                       c.) 3                         d.) 3.5                     e.) None of these

Explanation: Answer is C

Part filled by (X + Y + Z) in 1 hour =  =

∴ the three pipes will fill the tank in 3 hours.