Correct. The length of the cut, 20 in., is a hypotenuse of a right triangle. Call the side length of the square x. Use the Pythagorean Theorem:

$\displaystyle{{x}}^{{2}}+{{x}}^{{2}}={{20}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{2}{{x}}^{{2}}={400}\usepackage{cmbright}\Huge\$

$\displaystyle{{x}}^{{2}}={200}\usepackage{cmbright}\Huge\$

$\displaystyle{x}=\sqrt{{200}}\approx{14.1}\usepackage{cmbright}\Huge\$