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Question

Pipes A and B can fill an entire tank in 8 hours and 12 hours, respectively. The water tank is one-fourth full. If both the pipes are opened together, then how long will it take to fill the remaining part of the tank?

This question was previously asked in
RRB ALP 2025 CBT 2 Mechanic Motor Vehicle Question Paper (28-Jul-2026) (Shift 1)
The correct answer is

3 hours 36 minutes

To solve the given problem, let's break down the steps required to find the time taken to fill the remaining part of the tank when both pipes A and B are used together. Here's a detailed explanation:

  1. Determine the rates of pipes A and B:
    • Pipe A can fill the entire tank in 8 hours. Therefore, the rate of Pipe A is: \(\frac{1}{8}\) tank per hour.
    • Pipe B can fill the entire tank in 12 hours. Therefore, the rate of Pipe B is: \(\frac{1}{12}\) tank per hour.
  2. Find the combined rate when both pipes are opened together:
    • Combined rate of both pipes = \(\frac{1}{8} + \frac{1}{12}\).
    • To add these fractions, find a common denominator. The least common multiple of 8 and 12 is 24.
    • Convert each fraction: \(\frac{1}{8} = \frac{3}{24}\) and \(\frac{1}{12} = \frac{2}{24}\).
    • Add the fractions: \(\frac{3}{24} + \frac{2}{24} = \frac{5}{24}\).
    • Thus, both pipes together fill \(\frac{5}{24}\) of the tank per hour.
  3. Determine the remaining part of the tank to be filled:
    • The tank is initially one-fourth full, so the remaining part to fill is 1 - \(\frac{1}{4} = \frac{3}{4}\) of the tank.
  4. Calculate the time required to fill the remaining \(\frac{3}{4}\) of the tank using both pipes:
    • If the combined rate is \(\frac{5}{24}\) tank per hour, then the time required to fill \(\frac{3}{4}\) tank is: \(\frac{\frac{3}{4}}{\frac{5}{24}} = \frac{3}{4} \times \frac{24}{5} = \frac{72}{20} = 3.6\) hours.
    • Convert 3.6 hours to hours and minutes: 3.6 hours is 3 hours and 0.6 of an hour, i.e., \(0.6 \times 60 = 36\) minutes.
  5. Conclusion:
    • The time taken to fill the remaining part of the tank with both pipes open is 3 hours and 36 minutes.

The correct answer is: 3 hours 36 minutes.

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Similar Questions

  1. Three pipes A, B and C can fill a tank in 12 hours, 18 hours and 24 hours, respectively. A leak at the bottom can empty the full tank in 36 hours. If all the pipes are opened together, in how many hours will the tank be filled? (Round off your answer to two decimal places.)

  2. A pump can fill a tank in 3 hours. Due to a leak in the tank, it takes 4.5 hours to fill the tank. In how much time can the leak empty the full tank if no other entry or exit routes are open?

  3. Two pipes, when working one at a time, can fill a cistern in 3 hours and 4 hours, respectively while a third pipe can drain the cistern empty in 8 hours. All the three pipes were opened together when the cistern was 1/12 full. How long did it take for the cistern to be completely full?

  4. Two valves A and B can fill a sump in \(37\frac{1}{2}\) minutes and 45 minutes respectively. Both valves are opened. The sump will be filled in just 30 minutes, if valve B is turned off after?

  5. One pipe can fill an empty cistern in 4 hours while another can drain the cistern when full in 10 hours. Both the pipes were turned on when the cistern was half-empty. How long will it take the cistern to be full?

  6. A pipe, working at full speed, can fill an empty cistern in 1 hour. However, during the first hour it worked at one-twelfth of its capacity, during the second hour at one-ninth of its capacity, during the third hour at one-sixth of its usual capacity, during the fourth hour at one- fourth of its usual capacity and during the fifth hour it was only one-third as efficient as it was supposed to be. A second pipe also displayed similar performance, but if it worked at full speed would have filled the empty cistern in 2 hours. Together with a drain pipe that drained water out of the tank at a constant rate, the empty cistern could be filled in 5 hours, all the three pipes working concurrently. How many hours will it take the drain pipe to empty the filled cistern if no other pipe was functioning during the time?

  7. Pipes A and C can fill an empty cistern in 16 and 24 hours respectively while Pipe B can drain the filled cistern in 12 hours. If the three pipes are turned on together when the cistern is empty, how many hours will it take for the cistern to be full?

  8. Pipes A and C can fill an empty cistern in 32 and 48 hours, respectively while pipe B can drain the filled cistern in 24 hours. If the three pipes are turned on together when the cistern is empty, how many hours will it take for the cistern to be 2/3 full?

  9. One of the two inlet pipes works twice as efficiently as the other. The two, working alongside a drain pipe that can empty a cistern all by itself in 8 hours, can fill the empty cistern in 8 hours. How many hours will the less efficient inlet pipe take to fill the empty cistern by itself?

  10. Two pipes fill a tank when working individually in 25 and 40 hours, respectively while a third pipe can drain the filled tank in 16 hours. If all the three pipes are turned on at the same time when the tank is empty, how long will it take to fill the tank completely?

Important Questions from Pipe and Cistern

  1. A pipe can fill a cistern in 20 minutes where as the cistern when full can be emptied by a leak in 28 minutes. When both are opened, The time taken to fill the cistern is:

  2. ‘A’ pipe can empty a tank in 20 minutes. The second pipe ‘B’ has a diameter twice as that of ‘A’. If both A & B pipe are attached to the tank how much time will be required to empty the tank?

  3. Pipes A and B can empty a full tank in 16 hours and 24 hours, respectively. Pipe C alone can fill the empty tank in 4 hours. If A, B and C are opened together, the tank will be 35% full after :

  4. Pipes A and B can fill a tank in 36 minutes and 45 minutes, respectively. Both these pipes were opened simultaneously. After 20 minutes, a leak at the bottom of the tank was spotted which was immediately sealed. The tank was full in another 15 minutes. The leak alone can empty the full tank in:

  5. A cistern has a leak which would empty it in 6 hours. A tap is turned on which admits 10 litres of water per minute into the cistern. When it is full it is now emptied in 10 hours. What is the capacity (in litres) of the cistern?

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