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"Common Core Standard 5.NF.1 Add and subtract fractions with unlike denominators (including mixed numbers) by replacing given fractions with equivalent fractions in such a way as to produce an equivalent sum or difference of fractions with like denominators."
How to add fractions:

If the denominators are the same simply add the numerators, but don't add the denominators. For example: 2⁄4+ 1⁄4= 3⁄4

Step 1.If the denominators are not the same find a LCM, or least common multiple, for the denominators (bottom)

The LCM is the smallest multiple of both denominators. For example: 3/4 and 2/3 have the LCM of 12 because both 3 and 4 divide evenly into 12.

Step 2. Once you decide your LCM, multiple each fraction by the following value 

Step 2a.Take your denominator of the fraction, divide this number into the LCM 

Step 2b. Take the number from step 2a and create a fraction with a value of one

Step 3. Multiply the top (numerator) and bottom (denominator) by the fraction created in step 2b.  You now have two fractions with like denominators.

Step 4.  Add the numerators only.  Keep the like denominators.

Step 5. Simplify or reduce the final answer.

Step by step instructions adding fractions
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List of Lowest Common Multiples of frequently used numbers in math fraction problems.

Lowest common multiple 3 and 4 =12

8 and 12 = 24

6 and 8 = 24

4 and 6 = 12

12 and 15 = 60

8 and 14 = 56

4 and 14 =28

Finding the lowest common multiple is essential for adding fractions with unlike denominators. The LCM calculator below may help.

How to Use LCM Calculator
1. Total Number equals the number of fractions you are adding together. If you are working with 2 fractions it is 2, if you have 3 fractions it is 3

2. Next type in each denominator at the insert numbers

3. Click Go and the LCM is calculated

fraction 9/12 +8/12 = 17/12

Adding Fractions with unlike Denominators

divide each denominator
finding lcm
Step 4. Simpify or reduce the final answer.
simplyfy for a mixed number
Step 2. Divide each denominator into the LCM and create a fraction with a value of one. For example, 
12  ÷4= 3 therefore, becomes 3/3 and 12÷3 =4 therefore, becomes 4/4
Step 1. Because the denominators are different you need a LCM, lowest common multiple, which in this example is 12

How to add fractions with unlike denominators

Step 3. Add the numerators but do not add the denominators
​In this example we created a mixed number by dividing 17 by 12 which gives you the mixed number 1 5/12 however, you don't have to simplfy into a mixed number.

Sounds confusing but it is really not that bad. The example below will help.
Solve   3/4 + 2/3
Help, I need help with adding fractions that have different denominators. Follow these steps in order to add fractions.