1 : 3 = 2 : 6 = 3 : 9 = 4 : 12 = 5 : 15 = 6 : 18 = 7 : 21.... All of these ratios show the same relationship. Simplifying ratios to the simplest form can be helpful when solving problems that deal with ratios. You can simplify a ratio the same way that you simplify a fraction.
