An automobile radiator contains 20 l of antifreeze and water. this mixture is 20% antifreeze. how much of this mixture should be drained and replaced with pure antifreeze so that the mixture will be 50% antifreeze.

Try this solution (this suggested way is not short, but pure):
1. to make up the scheme of changes; 2. to make up the system of the equations according to the scheme.

Note, that antifreeze is marked by black in the picture.