Two pipes can fill a tank in 20 min. and 30 min. respectively. An out let pipe can empty 340 l of water per minute. If all the three pipes are opened it filled in 1hr 20min. Find the  capacity of the tank.

if all three pipes opened it takes 80 min to fill the tank

$\frac{1}{20}+\frac{1}{30}-\frac{1}{x}=\frac{1}{80}$

$\frac{1}{x}=\frac{1}{20}+\frac{1}{30}-\frac{1}{80}$

$\frac{1}{x}=\frac{12+8-3}{240}\to \frac{1}{x}=\frac{17}{240}$

the leakage pipe empty the tank in $\frac{240}{17}min$

if the leakage pipe empty 340 lit/minute

capacity of the tank=340$\times \frac{240}{17}=4800 lt$