Q: You have two long planks between which is trapped a load of gunge with a length of one unit (pictured). You then move one plank down one unit and then push it back up again. You repeat this six times. How long is the gunge now?
Reveal the solution
Moving the plank up from the position shown on the right it will smear the gunge two units up after the first move. And the same thing will happen next time, moving it three units up, and so on. So after six repetitions it will reach seven units up.