Answer
10.7ft
Work Step by Step
1. This works because the pole-shadow system creates similar triangles. You need to assume that both poles form 90 degree angle with the ground, and the Sun will create shadows with the same angle. AA (angle angle) is enough to prove similar triangles.
$\frac{30ft}{8ft}=\frac{40ft}{x}$
then $x=\frac{40\times 8}{30}=10.7ft$
