We can only add and subtract fractions that have the same bottom numbers. If fractions have the same bottom numbers, we can add them by just adding the top numbers and and keeping the original bottom number.
If they don't have the same bottom number, we have to find the least common denominator for those fractions which is simply the least common multiple of the two or how ever many fractions there are.
For example, to add 1/2 and 1/3, we find the least common
denominator, which is 6, and we want to get a 6 in the denominator.
When we have the same denominator, just add
the numerators and keep the original denominator.