| A cistern can be filled by two pipes A and B in 20 and 30 min, respectively. Both the pipes are opened together when the cistern is empty but after some time, A is stopped and the cistern is filled in 18 min in all. The time (in min) after which A is stopped, is |
A) 6
B) 8
C) 9
D) 12
Correct Answer: B
Solution :
| Let the time after which the pipe A was stopped be x min. |
| Then, \[\frac{x}{20}+\frac{18}{30}=1\]\[\Rightarrow \]\[\frac{3x+36}{60}=1\] |
| \[\Rightarrow \] \[3x=60-36\] |
| \[\therefore \] \[x=\frac{24}{3}=8\,\min \] |
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