Two pipes A and B can fill a cistern in 37,5 minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the B is turned off after:

Answers (1)

Let B be turned off after 'x' minutes. Then, Part filled by (A+B) in x min. + Part filled by A in (30-x) min =1 

\therefore x\left(\frac{2}{75}+\frac{1}{45}\right)+(30-x) \cdot \frac{2}{75}=1
\begin{array}{l} \Rightarrow \frac{11 x}{225}+\frac{60-2 x}{75}=1 \\ \Rightarrow 11 x+180-6 x=225 \\ \Rightarrow x=9 \end{array}​​​​​​​

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