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 170 and 155 is 15.

Next, we need to find the average of the two numbers. The average of 170 and 155 is (170+155)/2 = 162.5.

Now, we can calculate the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 170 and 155 is (15/162.5) x 100 = 9.23%.

Therefore, the percentage difference between 170 and 155 is 9.23%. This means that 155 is 9.23% less than 170.