let x = Toby's current test average.
Then: x + .10x would get you to an average that is is 10% greater.
Example: Lets say your current test score average is 80. 10% of 80 is 8. So 88 represents a 10% increase compared to 80.
Or, think of what 110% of 80 would be: 1.1(80) = 88
