A <em>unit </em>rate is the change in some quantity per <em>one</em> of another quantity. One common unit rate is <em>miles per hour</em>, which measure the number of miles traveled in <em>one hour</em>. Here, we're asked for the unit rate of plant growth per year, and we're told that at 16 years old, the plant measures 12 inches. Assuming this growth is <em>linear</em> (that is, it grows by the same amount each year), we can divide each quantity by the number of years (16) to find a unit rate of 12/16 = 3/4inch per year.
