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.