To find the percentage difference between two numbers, we first need to calculate the absolute difference between them. In this case, the absolute difference between 390 and 450 is 60 (450 - 390 = 60).

Next, we need to find the average of the two numbers. The average of 390 and 450 is (390 + 450) / 2 = 420.

Now, we can calculate the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 390 and 450 is (60 / 420) x 100 = 14.29%.

Therefore, the percentage difference between 390 and 450 is 14.29%. This means that the second number (450) is 14.29% greater than the first number (390).