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Question

A pipe can fill a tank in 5 hours, and another pipe can empty the same tank in 10 hours. How long will it take for both pipes to fill the tank when working together?

This question was previously asked in
RRB ALP 2025 CBT 2 Wiremen Question Paper (28-Jul-2026) (Shift 2)
The correct answer is
10 hours

Solving Pipe Filling and Emptying Problems

This problem involves calculating the time taken to fill a tank when one pipe fills it and another empties it simultaneously.

Calculating Filling Pipe Rate

The first pipe fills the tank in 5 hours. Its filling rate is the amount of the tank it fills per hour.

  • Rate of filling pipe = \(\frac{1}{5}\) tank per hour.

Calculating Emptying Pipe Rate

The second pipe empties the tank in 10 hours. Its emptying rate is the amount of the tank it empties per hour.

  • Rate of emptying pipe = \(\frac{1}{10}\) tank per hour.

Combined Work Rate

When both pipes work together, the net rate at which the tank fills is the filling rate minus the emptying rate.

  • Net filling rate = (Rate of filling pipe) - (Rate of emptying pipe)
  • Net filling rate = \(\frac{1}{5} - \frac{1}{10}\)
  • To subtract, find a common denominator, which is 10:
  • Net filling rate = \(\frac{2}{10} - \frac{1}{10} = \frac{1}{10}\) tank per hour.

This means that when both pipes are open, \(\frac{1}{10}\) of the tank is filled every hour.

Total Time Calculation

To find the total time required to fill the tank, divide the total work (filling 1 tank) by the net filling rate.

  • Time = \(\frac{\text{Total Work}}{\text{Net Filling Rate}}\)
  • Time = \(\frac{1 \text{ tank}}{\frac{1}{10} \text{ tank per hour}}\)
  • Time = \(1 \times \frac{10}{1}\) hours
  • Time = 10 hours.

Therefore, it will take 10 hours for both pipes to fill the tank when working together.

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Similar Questions

  1. Three pipes of diameters 2 cm, 3 cm and 4 cm are running together to fill a cistern. The smallest pipe alone can fill the cistern in 232 minutes. The amount of water flowing in each pipe is proportional to the square of its diameter. The time (in minutes) taken by all pipes running together to completely fill the cistern is:
  2. A cistern can be filled by a tap in 5 hours and emptied by an outlet pipe in 9 hours. How long will it take to fill the cistern if both the tap and the pipe are opened together?
  3. Tap A can fill an empty swimming pool in 10 h. Tap B can fill it in 15 h. How much time will the two taps take to fill the empty pool together?
  4. Two taps can fill an empty cistern in 8 min and 20 min, respectively. However, together, they take 30 min to fill it because of a leak. How much time will the leak take to empty a full cistern?
  5. Hose A and B can fill a pool in 5 and 10 minutes respectively. After 2 minutes, hose B gets blocked. How much time will hose A take to fill the remaining pool?
  6. A certain type of taps can fill a tank in 10 hours each. If there is a leakage that can empty the full tank in 4 hours, then how many taps are required to fill the tank in 4 hours?
  7. Two taps can fill an empty cistern in 8 min and 20 min, respectively. However, together, they take 30 min to fill it because of a leak. How much time will the leak take to empty a full cistern?
  8. Pipes A, B and C are attached to an empty cistern. While the first two can fill the cistern in 4 and 10 hours, respectively, the third can drain the cistern, when filled, in 6 hours. If all the three pipes are opened simultaneously when the cistern is half-full, how many hours will be needed to fill the cistern?
  9. Two pipes A and B can fill an empty cistern in 18 and 27 hours, respectively. Pipe C can drain the entire cistern in 45 hours when no other pipe is in operation. Initially, when the cistern was empty Pipe A and Pipe C were turned on. After a few hours Pipe A was turned off and Pipe B was turned on instantly. In all, it took 55 hours to fill the cistern. For how many hours was Pipe B turned on?
  10. A pipe can fill a sump with water in 2 hours. Because of a leak, it took $2 \frac{1}{3}$ hours to fill the sump. The leak can drain all the water of the sump in:

Important Questions from Pipe and Cistern

  1. Two pipes A and B can independently fill a tank completely in 20 and 30 minutes respectively. If both the pipes are opened simultaneously, how much time will they take to fill the tank completely?

  2. The compound interest on Rs. 64,000 for 3 years, compounded annually at 7.5% p.a. is

  3. A water tank can be emptied in 40 minutes by a pipe of d 'diameter, so how long will it take for a 2d diameter pipe to be emptied?

  4. Two pipes X and Y can fill an empty tank in 16 hours and 20 hours respectively. Pipe Z alone can empty the completely filled tank in 25 hours. Firstly both pipes X and Y are opened and after 6 hours pipe Z is also opened. What will be the total time (in hours) taken to completely fill the tank?

  5. Pipe J can fill a tank in 48 hours and pipe K can fill the same tank in 72 hours. If both the pipes are opened alternately for one hour each and pipe K is opened for the first hour, then in how much time the tank will be full?

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