To find the percentage difference between two numbers, we first need to calculate the absolute difference between them. In this case, the absolute difference between 81 and 100 is 19 (100 - 81 = 19).

Next, we need to find the average of the two numbers. The average of 81 and 100 is (81 + 100) / 2 = 90.5.

Now, we can calculate the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 81 and 100 is (19 / 90.5) x 100 = 20.99%.

Therefore, the percentage difference between 81 and 100 is approximately 20.99%. This means that 81 is about 20.99% less than 100.