Now that you know how to find common numbers, let's apply that skill to fractions. Common numbers can help us find the least common denominator in a set of fractions.
Take a look at these two fractions:
These fractions have different denominators: 3 and 4. If we want to add or subtract fractions with different denominators, we first need to find the least common denominator.
Remember that when you're looking for the least common denominator of two numbers, you're looking for the smallest number that they can both fit into. We find the least common denominator by asking, "What's the first common number for the denominators?" What is the first common number for 3 and 4? It's 12. It's the first common number, so it's the lowest common denominator.
To add the fractions together, we need to make their denominators match. Let's look at the first fraction, 
. Remember that our least common denominator is 12. How do we turn 3 into 12? We multiply by 4. Since we're multiplying the denominator by 4, we also need to multiply the top number (numerator) by 4 to make it match. What is 2 times 4? It's 8. So we change 
into 
.
Now let's look at the second fraction, 
. How do we turn 4 into 12? We multiply by 3. Since we're multiplying the denominator by 3, we also need to multiply the top number (numerator) by 3 to make it match. What is 1 times 3? It's 3. So we change 
into 
.
Let's look at the fractions now. We have 
and 
. The denominators are the same, so now we can add or subtract them. If we add them together, we get 
. If we subtract, we get 
.
