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A cistern can be filled by two pipes in 20 and 30 minutes respectively. Both pipes being opened, when must the first pipe be turned off so that the cistern may be filled in 10 minutes more?
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- after 10 minutes
- after 12 minutes
- after 20 minutes
- after 8 minutes
- after 10 minutes
Correct Option: D
As per the given in question ,
| In 1 minute both pipes can fill = | + | part of the cistern | ||
| 20 | 30 |
| In 10 minutes, second pipe can fill = | × 10 = | part | ||
| 30 | 3 |
| Cistern filled by both pipes = 1 - | = | ||
| 3 | 3 |
| ∴ Time taken by both the pipes to fill 2/3 part of cistern = | = 8 minutes | |
| 3 |
Therefore, the first pipe can be turned off after 8 minutes.
