The rate for 18:4 divided by 2:2 would be 9:2. This is because 18 divided by 2 is 9 and 4 divided by 2 is 2. The unit rate for 9:2 is 4.5:1. This is because you must divide 2 by 2 in order to get 1, and what you do to the second part you must do to the first part. 9 divided by 2 is 4.5.