One of the main rules for fractions is that you can only add and subtract fractions that have the same denominator.
So the question becomes, "what do we do when they don't have the same denominator?"
Easy. Change them so they do.
Example 1:

+ 

The first thing we need to do is think what we are going to change the denominators to. To do this we find the Lowest Common Multiple (LCM) of 3 and 4.
All this means is we want to know the first number that is in both the 3 and 4 times tables. If we write them out, we find that this is 12.
Step 1: Change the first fraction to a number over 12. This is done by multiplying both the numerator and denominator by 3

= 

Step 2: Change the second fraction to a number over 12. This is done by multiplying both the numerator and denominator by 4

= 

We can now rewrite our sum and work it out.

+ 

= 

+ 

= 
