How to Subtract Fractions

In math, learning to subtract fractions is essential. You can subtract two fractions if they have a common denominator but to find this common denominator you also need to know about least common multiples (LCM). The LCM is the smallest number that the denominators of each fraction will divide into evenly.

Difficulty: Easy

Instructions

1. Subtract 1/5 from 3/4. Rewrite this as 3/4 - 1/5 and solve.

2. Find the common denominator by determining the LCM of the two denominators of the fractions. These denominators are 4 and 5.

3. Determine the LCM of 4 and 5 by using the factorization method. The prime factors for 4 are 2, 2. Likewise, you find that for 5 the prime factor is 5. The LCM is then 2x2x5=20 (see Resources).

4. Use the LCM 20 to find equivalent fractions for 3/4 and 1/5. Multiply the numerator and denominator of 3/4 by 5 to get the equivalent fraction 15/20. Similarly find the equivalent fraction for 1/5 by multiplying by 4 and you get 4/20. Now you have common denominators for the fractions in Step 1.

5. Subtract the fractions 15/20 - 4/20 from Step 4 and the answer is 11/20.

Tips & Warnings

• You can subtract mixed numbers as well but first you need to convert the mixed numbers into improper fractions and then continue from Step 2 to Step 5 in this tutorial to find the answer.
• It is easier to use the prime factorization method to find the LCM when the denominators of the fractions are large (see Resources).

Resources

• Find information here on prime factorization and LCM.

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[stevemar2]

on 10/17/2009 Nice explanation of what is a difficult topic for some students. I myself have written math-related eHow articles too, so I enjoyed reading this.