RAY: "My car's cooling system holds 12 quarts."

"Right now it has four quarts of antifreeze, and eight quarts of water. How much of this mixture do I have to drain out and replenish with straight antifreeze in order to get a 50–50 mixture?"

I thought I would add a "Part B" to this puzzler. How much of this mixture do I have to drain out if I'm adding antifreeze that's already been mixed 50–50 to get a 50–50 mix?