Simplify the ratio 21 to 27?

To simplify the ratio 21 to 27, you need to find the greatest common divisor (GCD) of the two numbers, which is 3. Divide both numbers by 3 to simplify the ratio: 21 ÷ 3 = 7 and 27 ÷ 3 = 9. Therefore, the simplified ratio of 21 to 27 is 7 to 9.