In order to add or subtract fractions, the bottom numbers (denominators) of both fractions must be the same or common. The bottom number tells you the size of the pieces.
For example, 3/4 and 2/4 both share a common denominator of 4. An example of uncommon denominators would be 7/8 and 7/9. While the numerators (top numbers) are the same, the denominators are different 8 and 9. Remember, the number on the bottom is the denominator, and it is what tells you the size of the pieces.
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