A cistern has leak which would empty it in 10 hours. A tap is turned on which admits 10 liters a minutes into the cistern and it is now emptied in 15 hours. How many liters does the cistern hold ?

Tap can fill the cistern in ‘x’ hours

$\begin{array}{l}\frac{1}{x}-\frac{1}{10}=-\frac{1}{15}\\ \frac{1}{x}=\frac{1}{10}-\frac{1}{15}\\ \frac{1}{x}=\frac{6-4}{60}\\ \frac{1}{x}=\frac{2}{60}\\ \frac{1}{x}=\frac{1}{30}\end{array}$

Tap can fill the cistern in 30 hours

Capacity of  tank $\Rightarrow$ Tap can fill 10 liters per minutes

in  30 hours  $\Rightarrow 30\times 60\times 10$

$\Rightarrow 18000liters$