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A cistern can be filled by a tap in 3 hours while it can be emptied by another tap in 8 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?


  • a)
    4.8 hr
  • b)
    2.4 hr
  • c)
    3.6 hr
  • d)
    1.8 hr
Correct answer is option 'A'. Can you explain this answer?
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A cistern can be filled by a tap in 3 hours while it can be emptied by...
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A cistern can be filled by a tap in 3 hours while it can be emptied by...
Given:
- Time taken to fill the cistern by one tap = 3 hours
- Time taken to empty the cistern by another tap = 8 hours

To find:
- Time taken to fill the cistern when both taps are opened simultaneously

Explanation:
Let's assume that the capacity of the cistern is C liters.
Let's also assume that the rate of filling the cistern is R liters per hour.

Rate of filling by the first tap:
- The first tap fills the cistern in 3 hours.
- Therefore, the rate of filling by the first tap = C/3 liters per hour.

Rate of emptying by the second tap:
- The second tap empties the cistern in 8 hours.
- Therefore, the rate of emptying by the second tap = C/8 liters per hour.

Total rate when both taps are opened simultaneously:
- When both taps are opened simultaneously, the net rate of filling the cistern can be calculated by subtracting the rate of emptying from the rate of filling.
- Net rate = (C/3) - (C/8) liters per hour

Time taken to fill the cistern:
- Time taken to fill the cistern can be calculated by dividing the total capacity of the cistern by the net rate of filling.
- Time taken = C / [(C/3) - (C/8)] hours

Simplifying the expression:
- Taking the LCM of 3 and 8, we get 24 as the denominator.
- Time taken = C / [(8C - 3C) / 24]
= C / [(5C) / 24]
= 24C / 5C
= 24/5 hours
= 4.8 hours

Conclusion:
Therefore, the cistern will get filled in 4.8 hours when both taps are opened simultaneously. Hence, the correct answer is option 'A' (4.8 hr).
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