Aladder 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?

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

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