Answer
12 ft
Work Step by Step
Let $x$ be the length of shadow of the 30-ft tree.
When we draw the figure of trees and their shadows, we get two similar triangles. As the triangles are similar, sides corresponding to angles having equal measures are proportional.
$\implies \frac{20}{30}=\frac{8\,ft}{x}$ or $x=\frac{8\,ft\times30}{20}= 12\,ft$
