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Question

Two taps, A and B, can fill a cistern in 6 hours and 7.5 hours, respectively. Both the taps are opened for 2 hours and then B is turned off. How much time will A take to fill the remaining cistern?

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

Calculating Tap Filling Time

This problem involves calculating the time taken to fill a cistern by two taps with different filling rates.

Step 1: Determine Individual Tap Rates

Find the rate at which each tap fills the cistern per hour.

  • Tap A fills the cistern in 6 hours. Rate of A = \(\frac{1}{6}\) cistern/hour.
  • Tap B fills the cistern in 7.5 hours (which is \(\frac{15}{2}\) hours). Rate of B = \(\frac{1}{15/2} = \frac{2}{15}\) cistern/hour.

Step 2: Calculate Combined Rate and Work Done

Calculate how much work both taps complete when open together.

  • Combined Rate (A + B) = Rate A + Rate B = \(\frac{1}{6} + \frac{2}{15}\).
  • To add these fractions, find a common denominator (30): \((\frac{1 \times 5}{6 \times 5}) + (\frac{2 \times 2}{15 \times 2}) = \frac{5}{30} + \frac{4}{30} = \frac{9}{30} = \frac{3}{10}\) cistern/hour.
  • Work done in the first 2 hours = Combined Rate \(\times\) Time = \(\frac{3}{10} \times 2 = \frac{6}{10} = \frac{3}{5}\) of the cistern.

Step 3: Calculate Remaining Work

Determine the portion of the cistern left to be filled.

  • Total cistern capacity = 1.
  • Remaining Work = Total Capacity - Work Done = \(1 - \frac{3}{5} = \frac{2}{5}\) of the cistern.

Step 4: Calculate Time for Tap A to Fill Remaining Work

Find the time Tap A needs to fill the remaining portion alone.

  • Time = Remaining Work / Rate of A
  • Time = \((\frac{2}{5}) \div (\frac{1}{6})\)
  • Time = \(\frac{2}{5} \times 6 = \frac{12}{5}\) hours.
  • Converting to decimal: \(\frac{12}{5} = 2.4\) hours.

Therefore, Tap A will take 2.4 hours to fill the remaining cistern.

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Similar Questions

  1. Pipes A and B can fill a rectangular tank in 40 minutes and 120 minutes, respectively. Pipe C can empty the completely filled same tank in 240 minutes. If pipes A, B and C are opened at the same time, then how many minutes will it take to fill the tank?

  2. Tap A can empty a tank in 30 minutes. Tap B can empty it in 15 minutes. If both the taps operate simultaneously, how much time is needed to empty the tank?
  3. A tank has three pipes A, B and C. A and B are inlet pipes while C is an outlet pipe. Pipe A can fill the task in 15 minutes. Pipe B can fill it in 20 minutes. Pipe C can empty it in 30 minutes. The pipes are opened in the following cycle.
    Minute 1: Only A
    Minute 2: Only B
    Minute 3: Only C
    The cycle repeats till the tank is full. How much time would it take to completely fill the tank?
  4. 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:
  5. 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?
  6. Pipe A can fill a tank three times as fast as another pipe B. If the two pipes working together can fill the tank in 36 minutes, then in what time will the slower pipe alone fill the tank?
  7. Two pipes, A and B, can fill a water tank in 1 hour and \(1\frac{1}{2}\) hours, respectively. Pipe A is opened. After 15 minutes without closing it, pipe B is also opened. How much more time will both the pipes take to fill the tank?
  8. 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?
  9. A cistern has two inlets \(I_1\) and \(I_2\) which can fill it in 16 hours and 20 hours, respectively. An outlet can empty the full cistern in 12 hours. If all the three pipes are opened together in the empty cistern, how much time will they take to fill the cistern completely?
  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. A pipe can fill a tank in 4 hours, while a leak which is at one-fourth of the height of the tank from bottom can empty upto that part in 2 hours. If both are operated simultaneously and initially the tank is full, then when it will be one-fourth full?

  5. Two pipes can fill a tank in 10 hrs and 12 hrs, respectively, while the third can empty it in 20 hrs. If all the pipes are opened together, how much time will it take for the tank to be filled up?

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