problem: A tree is now 12 feet tall and grows 18 inches a year. In how many years will the tree be 36 feet tall?
solution: From 12 feet to 36 feet, the tree will grow 24 feet.  If it grows at a rate of 1.5 feet per year, then it will take the tree $\frac{24}{1.5}=\boxed{16}$ years to reach a height of 36 feet.