To add fractions, you must have a common denominator. Fractions that have common denominators are called like fractions. Fractions that have different denominators are called unlike fractions. To add like fractions, simply add the numerators and keep the same (or like) denominator. To add unlike fractions, first change all denominators to their lowest common denominator, also called the lowest common multiple or the denominator, the lowest number that can be divided evenly by all denominators in the problem. The numerators may need to be changed to make sure that the fractions are still equivalent to the originals. When you have all the denominators the same, you may add the numerators and keep the same denominator.