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 155 and 120 is 35.

Next, we need to find the average of the two numbers. The average of 155 and 120 is (155+120)/2 = 137.5.

Now, we can find the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 155 and 120 is (35/137.5) x 100 = 25.45%.

Therefore, the percentage difference between 155 and 120 is 25.45%. This means that the difference between the two numbers is 25.45% of their average value.