Students learn that a ratio is a comparison of two numbers or quantities. For example, if there are 2 shaded parts of a picture and 7 unshaded parts, the ratio of shaded parts to unshaded parts is 2 to 7. Note that the ratio 2 to 7 can also be written as 2:7 or 2/7. And since a ratio can be written as a fraction, it can be written in lowest terms. For example, the ratio 12 to 4 can be written as the fraction 12/4, which can be written in lowest terms as 3/1, or 3 to 1.