A straight ladder is leaning alongside the wall of a house. The ladder consists of rails 4.50 m long, joined by rungs 0.470 m long. Its bottom end is on solid although sloping ground so that the top of the ladder is around 0.780 m to the left of where it must be, and the ladder is unsafe to climb. You want to place a rock under one foot of the ladder to recompense for the slope of the ground.
What must the thickness of the rock be?