How Do You Find the Sum or Difference of Fractions?
To find the sum or difference of fractions, first find the lowest common denominator (LCD) of each fractions. Once you find the LCD, add or subtract the numerators to discover your answer.

Find the LCD
When the fractions have the same denominator, or bottom number, continue on to solve the problem. When the fractions have different denominators, convert them to equivalent fractions using the LCD rule. To find the LCD, calculate the least common multiple of all of the denominators. For example, you are given twothirds and foursixths. The least common multiple is six. Convert twothirds to an equivalent fraction by multiplying the numerator and the denominator by two. The equivalent fraction is foursixths.

Find the sum
Perform addition to find the sum of two or more fractions. When the denominators are the same, add the numerators, or top numbers, to calculate the answer. The denominator doesn’t change. For example, twothirds plus fourthirds equals sixthirds.

Find the difference
Subtract the smaller fraction from the larger fraction to calculate the difference. When the denominators are the same, subtract the smallest numerator from the largest. Write the answer over the original denominator for the correct answer. For example, fiveeighths minus threeeighths equals twoeighths.