Will is five feet tall. He casts a 3 feet shadow at the same time that a tree casts a 9 feet shadow. Use similar triangles to find the height of the tree.
the man is 5/3 as big as his shadow
so, the tree is 5/3 as big as its shadow
5/3 * 9 = 15
Or, without even knowing the ratio, note that since the tree's shadow is 3 times as long as the man's, the tree is 3 times as tall as the man: 15 ft.