Answer
17 ft
Work Step by Step
Since the diagonal forms a right triangle with the sides of the board, the Pythagorean theorem can be used to find the length of the diagonal. Substitute the length and width for the lengths of the legs and solve for the hypotenuse.
$8^2+15^2=d^2 \longrightarrow$ find the squares
$64+225=d^2 \longrightarrow$ add
$289=d^2 \longrightarrow$ find the square root of each side
$\sqrt {289}=\sqrt {d^2} \longrightarrow$ simplify
$17=d$
