In order to subtract fractions, you have to give them a common denominator.
18 1/2 can be rewritten as 18 6/12, as they are the same thing and 6/12 simplifies to 1/2. We make it 6/12 because we have to subtract it by 8 5/12, and the denominators are both 12.
So now we have 18 6/12 - 8 5/12, which gives us 10 1/12.
