It would be 12k.

Imagine making a u-turn at 10 mph. You start at 10 miles per hour, you leave at 10 miles per hour. Now imagine going down a road 10 mph, and you approach a roundabout that is headed towards you at 20 mph. (It is moving independently of your car and the road for this example.)

You enter the roundabout at 10 miles per hour, travel around it until you have travel 180 degrees, facing the direction you just came from. When you exit the roundabout, you will be going the 10 miles per hour that your car was already moving at, plus the 20 miles per hour that the roundabout was moving in the opposite direction, bringing your new speed to 30 miles per hour.

In this example, your car is a spaceship, and the roundabout is a moving gravity well, and the road, fixed in place, is space