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Question

A tap can fill an oil tank in 14 hours. After half the tank is filled, 7 more similar taps are opened. What is the total time taken to fill the oil tank completely?

This question was previously asked in
SSC CGL 2025 Tier 2 Paper 1 Question Paper (19-Jan-2026)
The correct answer is
7 hr 52 min 30 sec

Problem Analysis: Filling the Oil Tank

The problem asks for the total time to fill an oil tank. Initially, one tap fills the tank. After half the tank is filled, the number of taps increases. We need to calculate the time taken for both phases.

Step-by-Step Calculation

  1. Time for the First Half:

    A single tap fills the entire tank in 14 hours. Therefore, the time taken by one tap to fill half ($\frac{1}{2}$) the tank is:

    Time = Total Time $\times$ Fraction of Work = $14 \text{ hours} \times \frac{1}{2} = 7 \text{ hours}$.

  2. Rate of One Tap:

    The rate of work for one tap is the amount of tank it can fill per hour:

    Rate$_1 = \frac{1 \text{ tank}}{14 \text{ hours}} = \frac{1}{14}$ tank/hour.

  3. Taps for the Second Half:

    After the first half is filled (which took 7 hours), 7 more similar taps are opened. The total number of taps working together is:

    Total Taps = 1 (initial) + 7 (additional) = 8 taps.

  4. Combined Rate of Multiple Taps:

    The combined rate of 8 taps is:

    Combined Rate$_8 = 8 \times \text{Rate}_1 = 8 \times \frac{1}{14} = \frac{8}{14} = \frac{4}{7}$ tank/hour.

  5. Time for the Remaining Half:

    The remaining work is to fill the other half ($\frac{1}{2}$) of the tank. The time required for 8 taps is:

    Time$_2 = \frac{\text{Remaining Work}}{\text{Combined Rate}_8} = \frac{1/2 \text{ tank}}{4/7 \text{ tank/hour}} = \frac{1}{2} \times \frac{7}{4} \text{ hours} = \frac{7}{8} \text{ hours}.

  6. Convert Time to Hours, Minutes, and Seconds:

    To convert $\frac{7}{8}$ hours to minutes and seconds:

    • Minutes: $\frac{7}{8} \times 60 \text{ minutes} = \frac{420}{8} = 52.5 \text{ minutes}$.
    • This is 52 minutes and 0.5 minutes.
    • Seconds: $0.5 \times 60 \text{ seconds} = 30 \text{ seconds}$.

    So, the time taken for the second half is 52 minutes and 30 seconds.

  7. Total Time Calculation:

    The total time is the sum of the time taken for the first half and the second half:

    Total Time = Time$_1$ + Time$_2$ = 7 hours + 52 minutes 30 seconds.

    Total Time = 7 hours 52 minutes 30 seconds.

Final Answer

The total time taken to fill the oil tank completely is 7 hours 52 minutes 30 seconds.

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  5. Three pipes P, Q, and R can fill a cistern in 40 minutes, 80 minutes, and 120 minutes, respectively. Initially, all the pipes are opened. After how much time (in minutes) should the pipes Q and R be turned off so that the cistern will be completely filled in just half an hour?

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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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