Ratios and Rates · Superchallenge problem

A $30$-litre tub of drink is made of syrup and water in the ratio $1:4$. Some of the drink is poured out, and then the same amount of pure syrup is poured in, so there are $30$ litres again. Now the ratio of syrup to water is $1:2$. How many litres of drink were poured out?