To understand 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 120 is 30 (120 - 90 = 30).

Next, we need to find the average of the two numbers. The average of 90 and 120 is (90 + 120) / 2 = 105.

Now, we can calculate the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 90 and 120 is (30 / 105) x 100 = 28.57%.

This means that the second number (120) is 28.57% higher than the first number (90). Alternatively, we can say that the first number (90) is 23.08% lower than the second number (120).