Question

A ladder 20 ft long is resting against a vertical wall. The bottom of the ladder is 4 ft away from the wall. How high on the wall does the ladder reach?

Answer 1

Think of this like a triangle...

The height is 20 ft. The width is 4 ft.

It should look something like this:
----------/|
---------/-|
20ft--/--| x ft
-------/---|
------/__|
--------4ft