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 280 and 420 is 140 (420 - 280 = 140).

Next, we need to find the average of the two numbers. The average of 280 and 420 is (280 + 420) / 2 = 350.

Now, we can find the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 280 and 420 is (140 / 350) x 100 = 40%.

Therefore, the percentage difference between 280 and 420 is 40%. This means that 420 is 40% more than 280, or that 280 is 40% less than 420.