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A cistern can be filled with water by a pipe in 5 hours and it can be emptied by a second pipe in 4 hours. If both the pipes are opened when the cistern is full, the time in which it will be emptied is :
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- 9 hours
- 18 hours
- 20 hours
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20 1 hours 2
- 9 hours
Correct Option: C
According to the question ,
| Cistern filled in 1 hour = | part | |
| 5 |
| Cistern emptied in 1 hour = | part | |
| 4 |
When the both pipes are opened, simultaneously ;
| Cistern emptied in 1 hour = | - | = | = | part | ||||
| 4 | 5 | 20 | 20 |
∴ The time in which it will be emptied = 20 hours.
Using the given formula :
Here, p = 5, q = 4
| Required time = |  |  | hrs | |
| (p - q) |
| Required time = | = 20 hrs. | |
| 5 - 4 |
