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 91 and 88 is 3.

Next, we need to find the average of the two numbers. The average of 91 and 88 is (91+88)/2 = 89.5.

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

So, the percentage difference between 91 and 88 is (3/89.5) x 100 = 3.35%.

This means that the difference between 91 and 88 is 3.35% of the average of the two numbers.