Calculating the percentage difference betweeen two numbers is the trickiest to work out, and will take a little bit more practice than the other three. Again though, practice it a few times, get it stuck in your head, and you'll never look back.
If we use what difference is 35 from 84 as an example.
First, we need to work out the difference between the two numbers by subtracting the larger number from the smaller one.
Next we add the two numbers together, and divide them by two to get an average.
Then, we divide our original 49 by the 59.5 and mulitply by 100 to get our percentage.
You can see this is a bit trickier, but once you've done it a few times, you shouldn't forget how to do it. You can always come back here to see how it's done.