Answer:
The tank's capacity is 500 liters
Explanation:
Proportions
The tank initially contains 50 liters of water. The tank's capacity is more than that, as we'll show later.
We know after 20 minutes of leaking from the tank, 40 liters still remain. This means 10 liters have leaked in 20 minutes. The loss of water occurs at a constant rate or 10/20=0.5 liters per minute.
After 30 more minutes, the tank has lost 0.5*30 = 15 more liters of water and now has 40 - 15 = 25 liters remaining.
We are told those 25 liters correspond to 5% of the tank's full capacity. Thus, the full capacity of the tank is:

The tank's capacity is 500 liters