Answer
19 ft
Work Step by Step
The length of the ladder is the hypotenuse,
x = the height it reaches, opposite to the angle $72^{o}$.
$\displaystyle \sin 72^{o}=\frac{x}{20}$
$ x=20\sin 72^{o}\approx$19.0211303259$\approx$ 19 ft
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