This is an example of Pythagorean theorem. The equation for that is[tex]a^{2}+b^{2}=c^{2}[/tex] and so what you would do is substitute a with 80 and c with 100. That would make it [tex]80^{2}+b^{2}=100^{2}[/tex] .......anyways....the answer your looking for is 60ft.
