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All questions of Pipes and Cisterns for Commerce Exam

.

  • a)
    A
  • b)
    B
  • c)
    C
  • d)
    D
Correct answer is option 'D'. Can you explain this answer?

Notes Wala answered
(A + B)'s 1 hour's work =  = 9/60 = 3/20.
(A+C)'s hour's work =   = 8/60 = 2/15.
Part filled in 2 hrs = 
Part filled in 6 hrs = 
Remaining part = 
Now, it is the turn of A and B and 3/20 part is filled by A and B in 1 hour.
∴ Total time taken to fill the tank = (6+1) hrs = 7 hrs

A tap can fill a tank in 4 hours. After half the tank is filled, two more similar taps are opened. What is the total time taken to fill the tank completely?
  • a)
    1 hr 20 min
  • b)
    4 hr
  • c)
    3 hr
  • d)
    2 hr 40 min
Correct answer is option 'D'. Can you explain this answer?

Ishani Rane answered
Explanation :
A tap can fill a tank in 4 hours
= The tap can fill half the tank in 2 hours
Remaining part = 1/2
After half the tank is filled, three more similar taps are opened.
Hence, total number of taps becomes 4.
Part filled by one tap in 1 hour = 1/4
Part filled by four taps in 1 hour = 
4*1/4=1
i.e., 4 taps can fill remaining half in 40 minutes
total time taken = 2 hour + 40 minute = 2 hour 40 minutes

Two pipes A and B together can fill a cistern in 4 hours. Had they been opened separately, then B would have taken 6 hours more than A to fill the cistern. How much time will be taken by A to fill the cistern separately?
  • a)
    6 hours
  • b)
    4 hours
  • c)
    2 hours
  • d)
    3 hours
Correct answer is option 'A'. Can you explain this answer?

Arnav Sen answered

Given Information:
- Together, pipes A and B can fill the cistern in 4 hours.
- If opened separately, pipe B takes 6 hours more than pipe A to fill the cistern.

Let's Assume:
- Let the time taken by pipe A to fill the cistern separately be x hours.
- Therefore, the time taken by pipe B to fill the cistern separately would be (x + 6) hours.

Calculations:
- The combined rate of pipes A and B = 1/4 cistern per hour (since they can fill the cistern in 4 hours together)
- The individual rate of pipe A = 1/x cistern per hour
- The individual rate of pipe B = 1/(x + 6) cistern per hour
- According to the given information, the sum of individual rates of A and B is equal to their combined rate:
1/x + 1/(x + 6) = 1/4
- Solving the above equation, we get:
(x + 6 + x) / (x(x + 6)) = 1/4
(2x + 6) / (x^2 + 6x) = 1/4
8x + 24 = x^2 + 6x
x^2 - 2x - 24 = 0
(x - 6)(x + 4) = 0
x = 6 or x = -4

Conclusion:
- Since time cannot be negative, the time taken by pipe A to fill the cistern separately is 6 hours (option A).

One pipe can fill a tank four times as fast as another pipe. If together the two pipes can fill the tank in 36 minutes, then the slower pipe alone will be able to fill the tank in:
  • a)
    144 min
  • b)
    180 min
  • c)
    126 min
  • d)
    114 min
Correct answer is option 'B'. Can you explain this answer?

Mainak Saha answered
Suppose the slower pipe can fill the tank in x minutes.
Then the faster pipe can fill in x/4 minutes.
Part filled by the slower pipe in 1 min =1/x
Part filled by the faster pipe in 1 min =4/x
Part filled by the both the pipes in 1 min =1/x +4/x
 
Therefore, both the pipes can fill together in 36 minutes.
Part filled by both in 1 minute = 1/36.
1/x + 4/x = 1/36
5/x = 1/36
 x = 5 * 36 = 180

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