Explanation of Fractions

To add and subtract fractions, you need a common denominator. 
The denominator is the bottom number of a fraction. A common denominator simply means that they are both the same. 
To find a common denominator, compare the multiples of the numbers in the denominators. Remember, the common denominator is the same as a common multiple.

When dividing and multliplying there is no need for a common denomanter because in those problems your first method of working the problem is to cross multiply.