Loc: Bucks, England
Goes contrary to what you think is the obvious (9.2), but if you imagine the centre of the rollers being stationary, then the slab goes 9.4 one way, and the ground goes 9.4 the other way. So the plank moves approx 18.4 in relation to the ground.
Sometimes I get the feeling the whole world is against me, but deep down I know that's not true. Some smaller countries are neutral.