posted by Bob .
A straight ladder is leaning against the wall of a house. The ladder has rails 4.50 m long, joined by rungs 0.470 m long. Its bottom end is on solid but sloping ground so that the top of the ladder is 0.780 m to the left of where it should be, and the ladder is unsafe to climb. You want to put a rock under one foot of the ladder to compensate for the slope of the ground.
What should the thickness of the rock be?