A town's population grew from 400 to 500 over one year. What was the percent growth rate?

Percent growth is the change divided by the starting value. The change is $500 - 400 = 100$, and the starting value is 400, so $\frac{100}{400} = 0.25 = 25\%$, choice B. Choice A (20%) divides by the ending value, $\frac{100}{500}$, which uses the wrong base. Choice C (100%) uses the change itself, 100, as the percent. Choice D (125%) is the ending value as a percent of the starting value, $\frac{500}{400}$, which is the growth factor and includes the original 100%. Growth rate is measured against where the population started.