A vessel is 1/3 full of juice. After 30 more litres are poured in, it becomes 3/4 full. The capacity of the vessel is:

The 30 litres fill the gap between 1/3 and 3/4 of the vessel, i.e. 3/4 − 1/3 = 9/12 − 4/12 = 5/12 of it. So 5/12 of the capacity is 30 litres, giving capacity = $30\times 12/5=72$ litres. Check: 1/3 of 72 = 24 and 24 + 30 = 54 = 3/4 of 72.