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A, B and C are three pipes attached to a cistern. A and B can fill the cistern respectively in 20 and 30 minutes, while C can empty the cistern in 15 minutes. If A, B and C be kept open successively for 1 minute each, how soon will the cistern be filled?  
  • a)
    167 minutes
  • b)
    155 minutes
  • c)
    180 minutes
  • d)
    165 minutes
Correct answer is option 'A'. Can you explain this answer?
Most Upvoted Answer
A, B and C are three pipes attached to a cistern. A and B can fill the...
LCM of 15, 20 and 30 is 60.
Let the capacity of tank be 60 liters.
A and B fill the tank at a rate of 3 liters per minute and 2 liters per minute, respectively. C empties the tank at a rate of 4 liters per minute.
If opened successfully for 1 minute each, 1 liter will be filled in 3 minutes.
So, 55 liters will be filled in 55 × 3 = 165 minutes
After the 165th minute, tap A and B will fill the left capacity of 5 liters in 2 minutes.
So, the total time taken = 167 minutes
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Community Answer
A, B and C are three pipes attached to a cistern. A and B can fill the...
Understanding the Problem:
The problem involves three pipes - A, B, and C attached to a cistern. Pipe A can fill the cistern in 20 minutes, pipe B can fill it in 30 minutes, and pipe C can empty it in 15 minutes. The task is to determine how soon the cistern will be filled if pipes A, B, and C are kept open successively for 1 minute each.

Calculating the Efficiency of Each Pipe:
- Pipe A fills the cistern in 20 minutes, so its efficiency is 1/20.
- Pipe B fills the cistern in 30 minutes, so its efficiency is 1/30.
- Pipe C empties the cistern in 15 minutes, so its efficiency is -1/15 (negative because it empties the cistern).

Calculating the Combined Efficiency:
When all three pipes are open successively for 1 minute each, the net efficiency for each cycle is (1/20 + 1/30 - 1/15) = 1/60.

Calculating the Total Time:
Since the combined efficiency is 1/60, the time taken to fill the cistern is the reciprocal of this value, which is 60 minutes.
However, since the pipes are opened successively for 1 minute each, the cistern will be filled in 60 * 3 = 180 minutes.
Therefore, the cistern will be filled in 180 minutes.

Conclusion:
The correct answer is option (c) 180 minutes.
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A, B and C are three pipes attached to a cistern. A and B can fill the cistern respectively in 20 and 30 minutes, while C can empty the cistern in 15 minutes. If A, B and C be kept open successively for 1 minute each, how soon will the cistern be filled?a)167 minutesb)155 minutesc)180 minutesd)165 minutesCorrect answer is option 'A'. Can you explain this answer?
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