Answer
$13.2\ ft$
Work Step by Step
Step 1. Draw a diagram as shown in the figure, where $x$ is the height the ladder will reach.
Step 2. Using Pythagorean's Theorem in the right triangle, we have $x^2+15^2=20^2$ or $x^2=400-225=175$
Step 3. Discard the negative solution; we have $x=\sqrt {175}=5\sqrt 7\approx13.2\ ft$
