To find the percentage difference between two numbers, we first need to find the absolute difference between them. In this case, the absolute difference between 81 and 90 is 9.

Next, we need to find the average of the two numbers. The average of 81 and 90 is (81+90)/2 = 85.5.

Now, we can find the percentage difference by dividing the absolute difference by the average and multiplying by 100.

So, the percentage difference between 81 and 90 is (9/85.5) x 100 = 10.53%.

This means that the second number, 90, is 10.53% greater than the first number, 81.