Answer: B

A diagonal is the hypotenuse of a right triangle with legs $48$ and $20$. By the Pythagorean theorem, $d^2 = 48^2 + 20^2 = 2{,}304 + 400 = 2{,}704$, so $d = \sqrt{2{,}704} = 52$ inches.

Choice D is the value of $d^2$; the last step is to take its square root.

Why the other choices are wrong

• A. This subtracts the sides, but the Pythagorean theorem adds their squares.
• C. This adds the sides, but the diagonal is the square root of the sum of their squares.
• D. 2,704 is the square of the diagonal. Take the square root.