72:20 is the same as 18:5 but 18:5 is not the same as 5:18 when it's applied to two quantities. The order of the numbers is important. For example, if we say the ratio of boys to girls is 5:18 (implying 23 boys and girls, or some multiple of 23, like 4*23=92), we can also say that the ratio of girls to boys is 18:5, but we can't say that the ratio of girls to boys is 5:18. We can express the ratio as a proportion: 18/23 and 5/23 imply the ratio 18:5. In the example, if there are 92 girls and boys, 72 would be girls and 20 boys. The ratio is still 18:5, 18 girls to 5 boys.