Chapter 2 - Section 2.1 - The Distance and Midpoint Formulas - 2.1 Assess Your Understanding - Page 156: 64

$84.85\ \ \mathrm{ ft}$

The diagonal of the square is the hypotenuse of a right triangle whose legs are sides of the square. Apply the Pythagorean theorem. \begin{aligned}60^{2}+60^{2}&=d^{2}\\ 2\cdot 60^{2}&=d^{2}\\\\ d&=\sqrt{2\cdot 60^{2}}\\ &=60\sqrt{2}\\ &\approx 84.85\ \mathrm{ft}\end{aligned}