To understand the percentage difference between two numbers, we first need to understand what percentage means. Percentage is a way of expressing a number as a fraction of 100. For example, 50% means 50 out of 100 or 0.5 as a decimal.

Now, let's look at the percentage difference between 21 and 19. To find the percentage difference, we need to first find the absolute difference between the two numbers. Absolute difference means the difference without considering whether the numbers are positive or negative. In this case, the absolute difference between 21 and 19 is 2.

Next, we need to find the average of the two numbers. The average of 21 and 19 is (21+19)/2 = 20.

Now, we can find the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 21 and 19 is:

|21-19| / 20 x 100 = 2/20 x 100 = 10%

Therefore, the percentage difference between 21 and 19 is 10%. This means that 21 is 10% greater than 19.