Answer:
25.45%
Step-by-step explanation:
If we have two values a and b, then the percent difference is defined as

In other words, the percent difference is defined as the absolute value of the difference divided by their average.
Now in our case, the two numbers we have are 155 mph and 120 mph. Now the average of 155 and 120 is

The difference between the two numbers is

Therefore, the percentage difference is

which we evaluate to get

which is our answer!