.
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.
Solved, answered, explained and completed.
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Do not accept nothing from the post by @josgarithmetic,
since he interprets the problem's description incorrectly.