Right, so say you start the 'race' with the robber with 40 miles of a headstart (half an hour, hour is 80 miles... )

And he's travelling at 80 mph, and the cops are travelling at 90mph.

So, when are they equal?

When 80x + 40 = 90x.

Where x is the length of time , so x = 4.

So, after 4 hours after the cops have arrived the robber will have travelled 80*(4) + 40 = 360.

And the cops will have travelled 90 * 4 = 360 miles.