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1. A pipe can fill a tank in 3 hours. There are two outlet pipes from the tank which can empty it in 7 and 10 hours respectively. If all the three pipes are opened simultaneously, then the tank will be filled in -

A. 8 hours

B. 9 hours

C. 10 hours

D. 11 hours

Solution:

Net part filled in 1 hour

\begin{aligned} = \frac{1}{3} - (\frac{1}{7} + \frac{1}{10}) \\ = \frac{1}{3} - \frac{17}{70} \\ = \frac{19}{210} \end{aligned}

∴ The tank will be filled in

\frac{210}{19}

hours i.e.

\begin{aligned} =  11 \frac{1}{19} hours \\ ≅ 11 hours \end{aligned}

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