Fractions with like denominators have the same bottom number, such as 1/2 and 5/2. To add or subtract fractions, the denominators must be the same. Simply add the numerators and keep the same denominator. For example: 1/3 + 2/3 = 3/3, which simplifies to just 1.
Fractions with unequal bottom numbers are considered to have unlike denominators. For example: 2/3 and 2/4 have unlike denominators. To add or subtract fractions with unlike denominators, turn them into like denominators using the least common denominator (LCD). The LCD is the lowest number each of the denominators can divide into. The fractions 2/3 and 3/4 have denominators of 3 and 4. The LCD is 12 because both 3 and 4 can divide into that number. The numbers can also divide into 24, but it's more efficient to use the lowest number, which is 12.
Practice adding unlike fractions using an example, such as 3/4 + 1/6. First, find the LCD, which in this case is 12. Multiply the numerator of the first fraction by the quotient of lowest common denominator divided by the denominator of the first fraction: 12 divided by 4 is 3. Then 3 x 3 = 9, so the first fraction becomes 9/12.
In the case of 1/6, the equivalent fraction is 2/12. (Here 12 divided by 6 is 2. Then 2 x 1 = 2.) Now you can easily add the fractions: 9/12 + 2/12 = 11/12
Practice subtracting fractions with unlike denominators. Consider 1/2 - 1/3. Find the LCD, which is 6. The first fraction then becomes 3/6. (Here 6 divided by 2 is 3, and 3 x 1 = 3.) The second fraction, 1/3, becomes 2/6. (In this case, 6 divided by 3 is 2, and 2 x 1 = 2.)
Now subtract the results: 3/6 - 2/6 = 1/6.