[stevage]

Pragmatics is a big part of this.

If you introduced it with "Here's a logic problem..." then people will approach it one way.

But as specified, it's hard to know what is really being asked. If you are actually going to wash your car at the car wash that is 50 metres away, you don't need to ask this question.

Therefore the fact that the question is being asked implies that something else is going on...but what?