Adding and Subtracting Fractions
Key points to remember:
 You MUST have common denominators to add or subtract fractions
 You only add the numerators, the denominator stays the same.
With a Common Denominator:
Ex.: 3 + 2 = 5
8
8
8
Without Common Denominators:
If you have unlike denominators, you must find a common and make equivalent fractions with the new
denominator.
How to find a common denominator:
1. List the multiples of each denominator.
2. Find the smallest multiple they have in common. This is the Least Common Multiple (LCM)- it
will be your new denominator.
3. Make equivalent fractions with the new denominator.
Example : 1 + 3 = ?
4
5
Step 1: List the multiples of each denominator.
4: 4, 8, 12, 16, 20, 24, 28,…
5: 5, 10, 15, 20, 25, 30, 35,…
Step 2: Find the Least Common Multiple.
LCM = 20
Step 3: Make equivalent fractions with your new denominator.
(When making equivalent fractions, the rule is: Whatever you do to the top, do the same
thing to the bottom.” You can only multiply or divide to make your new fractions.)
x5
1 = 5
4
20
x5
x4
3 = 12
5
20
x4
Step 4: Do it (Add or Subtract based on your original problem using your new fractions.)
5 + 12 = 17
20
20
20