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.
"Ladies - never criticize your husband's taste. Remember, he chose you" - Mrs Worrier