³/₄ part of the tank is full of water. When 30 litres of water is taken out, the tank becomes empty. The capacity of the tank is -

Explanation:-

If tank has 4x liters of total capacity and its holds 3x liters of water and, if 30 liters of water is taken out, then tank becomes empty. It means 3x liters of water is taken out
3x = 30 liters
x = 10 liters
Capacity of tank
= 4x = 4 * 10 = 40 liters