Option 1, 8%, is incorrect because it is too close to the difference between the two percentages stated in the initial question (28%-20%) but doesn`t take into account how these percentages relate to each other, not just to Z.

Option 2, 9%, is incorrect for the same reason as Option 1. Both of these percentages are greater than the actual difference between X and Y when calculated appropriately.

Option 3, 10%, is the correct answer. With X being 20% less than Z and Y being 28% less than Z, the difference between X and Y in relation to Z is 10%. This means that Y is 10% less than X.

Option 4, 12%, is incorrect because this percentage is too high. This would mean that Y is 12% less than X, which is not accurate based on the percentages provided in the initial question.

The key to understanding the question is to realize that the percentages in the options relate to the relation between X and Y, not to Z.