For starters, determine what two things are changing, in this case, the question tells you it's the diameter and the year. Next, determine by how much these two are changing. For the year, subtract the most recent year by the oldest year, so 2005-1965=40 years. For the diameter, subtract the diameter of the most recent year by the diameter of the oldest year, so 251-248=3. Now, use the question to get your ratio. The question asks for "diameter of the tree per year," so you would divide the change in diameter by the change in year, so 3/40 inches per year would be your ratio.
