From first two sentences, we have this equation

    V(t) = 50 - a*t.


representing the current volume V(t) as a function of time, t, in minutes,

and "a" is the constant leaking rate in liters per minute.



At t= 20 minutes,  

    V(20) = 40 = 50 - a*20,


which gives  a =  =  = 0.5  liters per minute.



After further 30 minutes,  the volume of the water in the tank is

    V(50) = 40 - 0.5*30 = 40 - 15 = 25 liters.


So, 25 liters is 5% of the tank capacity.



Hence, the total capacity of the tank is   = 500 liters.



ANSWER.  The total capacity of the tank is   = 500 liters.