Cory was 100 pounds in the year 2000. This was 10% more than what he weighed in 1999. What did he weigh in 1999?

You start by finding out what 10% of 100 is & you can find this by setting up a ratio of 10/100 to x/100 & solve for x. It becomes clear that 10 is 10% of 100. So to find what he weighed in 1999, you would subtract 10 from 100 & that equals 90 pounds.