Question

Two pipes A and B can fill a cistern in $30$ min and $40$ min respectively, Both the pipes are turned on simultaneously. When should the second pipe be closed if the cistern is to be filled in $24$ min ?

• A. $6$ min
• B. $8$ min
• C. $10$ min
• D. $12$ min

Answer 1

The correct option is B $8$ min

Let the second pipe be closed after x minutes.

Then, $\frac{24}{30}+\frac{x}{40}=1$

$\Rightarrow \frac{96+3x}{120}=1\Rightarrow 3x=120-96=24\Rightarrow x=8min$