A brain teaser

Quick off the mark there Mr spoilsport, but some will struggle. E.g. a BBC egghead contestant failed to spot that speed * time equal distance...

Here's a brain teaser for you; WTF has this got to do with car maintenance?

Chris

LOL! Someone might want to check if the speedo is working ok.

OK lets make it a bit more complicated for you.

You are driving on a motorway at 70mph and at some point Q, you are joining a queue in front of you.

Assume the following:

Vehicles are equally distributed before the queue; each vehicle driving 70mph and occupying 50m of road space in a lane, including the vehicle itself.

After joining the queue, each vehicle is driving 10mph and occupying 10m of road space in a lane.

Assume instant transition from 70mph to joining the queue at 10mph.

As often observed, the queue joining point Q is moving slowly backward along the motorway.

What is the speed of the joining point Q backward along the motorway?

And yes, I have the answer to the problem.

In message , johannes writes

Is this before the multi-vehicle pile-up, or afterwards?

I know... The assumption of instant transition from 70mph to joining the queue at 10mph is just to make the problem more tractable and easier to solve. If there is a transition (braking) zone, it will probably be of some constant length, and hence the backward speed of the joining point Q will probably be the same, only that the joining point will not be sharply defined.