Answer
$15 \ \mathrm{ft}$
Work Step by Step
Draw a diagram of the problem like the one shown below and note by $x$ the distance the ladder will placed from the base of the house. Use the Pythagorean Theorem to determine $x$: $$
\begin{aligned}
x^2+20^2 & =25^2 \\
x^2 & =25^2-20^2 \\
x^2 & =625-400 \\
x^2 & =225 \\
x & =\pm\sqrt{225} \\
x&=\sqrt{225}\\
& =15 \mathrm{ft}.
\end{aligned}
$$ We took only the positive value since the distance must be positive.
The required distance is $15 \ \mathrm{ft}$.
