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 15 is 75 (90 - 15 = 75).

Next, we need to find the average of the two numbers. The average of 90 and 15 is (90 + 15) / 2 = 52.5.

Now, we can find the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 90 and 15 is (75 / 52.5) x 100 = 142.86%.

This means that the second number (15) is 142.86% less than the first number (90). Alternatively, we can say that the first number (90) is 242.86% more than the second number (15).