A cistern can be filled by two pipes in 30 and 40 minute respectively. Both the pipes are opened at once, When the first pipe be turned off so that the tank may be just filled in 18 minute? Class: 9

# A cistern can be filled by two pipes in 30 and 40 minute respectively. Both the pipes are opened at once, When the first pipe be turned off so that the tank may be just filled in 18 minute? Class: 9

1. A
12.5 min
2. B
13.5 min
3. C
14.5 min
4. D
16.5 min

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### Solution:

Let the first pipe be turned off after x minutes.
Work done by (first pipe + second pipe in 18 minutes) = 1
$⇒$ $\frac{x}{30}$ + $\frac{18}{40}$ = 1
= 1
4x = 120 $-$ 54
4x = 66
x = 16.5 min
So, option (4) is correct.

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