#### Answer

Shania made a mistake in simplifying $8\sqrt{20}$.
The correct answer is $9\sqrt5$.

#### Work Step by Step

Shania made a mistake in simplifying $8\sqrt{20}$.
The correct solution and answer is:
$8\sqrt{20} -7\sqrt5
\\=8\sqrt{4(5)}-7\sqrt{5}
\\=8\cdot 2\sqrt5 - 7\sqrt5
\\=16\sqrt5 - 7\sqrt5
\\=(16-7)\sqrt5
\\=9\sqrt5$