Answer
13.2 ft
Work Step by Step
Use the Pythagorean Theorem. Let x be the height the ladder reaches.
$20^2=15^2+x^2$
Simplify.
$400=225+x^2$
Subtract 225 from each side.
$400-225=225+x^2-225$
Simplify.
$175=x^2$
Take the square root of each side.
$\sqrt{175}=\sqrt{x^2}$
Simplify and round.
$13.2\approx x$
