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 90 and 60 is 30 (90 - 60 = 30).

Next, we need to find the average of the two numbers. The average of 90 and 60 is (90 + 60) / 2 = 75.

Now, we can find the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 90 and 60 is (30 / 75) x 100 = 40%.

This means that 60 is 40% less than 90, or that 90 is 40% more than 60. It's important to understand that percentage difference is a way to compare two numbers, and it can be useful in many different situations, such as comparing prices, test scores, or even the growth of a population over time.