Michael asked the following:
"When finding a percentage difference of two figures, e.g. 400 and 500,
"400-500 = 100, so 100/400x100 = 25%"
"But if you do 100/500x100 = 20%??"
Thank you for this, Michael! The answer is that it depends on what you want to know:
If you want to know how much more a larger number (500) is as a percentage of a smaller number (400) then use the first method - you correctly found that the difference (100) is a quarter (25%) of 400 (the smaller number).
If you want to know how much smaller a number (400) is than a larger one (500), the use the second technique - you correctly showed that the difference (100) is 1/5 (20%) of 500.