Hight of a Tree

Today, in math, we learned how to measure the hight of a tree based on its shadow, an object next to it, and the objects shadow.  Let's say the thingy standing next to the tree is 6 feet tall.  His shadow is 8 feet long.  The trees shadow is 24 feet long.  From all of that data, we could find the hight of the tree.  The answer would be................... 18 feet tall.