Using algebra to solve mixture problems

Hi,

 I have this problem that I am not able to solve because I do not know anything about mixtures.

  A car's radiotor contains 12 L of antifreeze at 25% concentration. How many liters must be drained and then replaced by pure antifreeze to bring the concentration to 50%?

 The answer is 4L but the book did not provide workings. Please show me the workings.

Thank you.