When two or more fractions have the same denominator (the number on the bottom).
The denominators must be the same before you can add or subtract the fractions.
You can find a common denominator by trying different multiples of the fractions.
Example: you can't add 1/3 and 1/2 as they are.
But when you multiply 1/3 by 2/2 you get 2/6
And when you multiply 1/2 by 3/3 you get 3/6
Now you can add 2/6 and 3/6 to get 5/6
So 1/3 plus 1/2 equals 5/6