Subtracting Fractions

The procedure of fraction addition is more or less the same as that used in the subtraction of fractions. This implies that if you have the know-how of adding fractions, you will have an easier time subtracting them. Subtraction of fractions is also one of the most common operations that we are likely to come across in many mathematical computations. All the same, if you have a problem subtracting fractions, there is absolutely no need to worry because what you need is only a few steps and procedure away.

To begin with, it is important to understand the structure of the fraction. A fraction can either be proper improper or mixed. In cases where you are required to perform subtraction of improper fractions, the first step should be to change it into an improper fraction first. A fraction has basically two numbers. One on top and the other at the bottom. The one on top is called the numerator, and the one at the bottom is the denominator. For example:

Given a/b, a is the numerator and b are the denominators.

Having understood the structure of a basic fraction, we shall consider two methods that can be used to subtract fractions basing on the number of fractions being subtracted. These methods require the understanding of the structure of a fraction.

Method of Common Denominator

Method of Least Common Denominator

  1. Method of Common Denominator

This method is well applicable when you are subtracting one fraction from another, meaning, you are dealing with two fractions only. Since by now we know the meaning of a denominator, it will also be-be helpful to understand the meaning of “Common Denominator.” What is a Common Denominator? When the denominators of the two fractions in question are the same or identical, we say that there exists a common denominator.

Therefore, the main aim of this method is to ensure that the denominators of the fractions are the same before performing the subtraction. When the denominators are not the same, subtraction cannot be done. How do we then ensure that the fractions have the same denominators? Multiply each fraction – the numerator and the denominator – by the denominator of the other fraction.

For illustration consider;

a/b – c/d = (a*d)/(b*d) – (c*b)/(d*b)

Once the denominators of the two fractions is now common, (b*d), the ‘new’ numerator are simply added.

Example; 1/2 – 1/3

(1*3)/(2*3) – (1*2)/(3*2) = 3/6 – 2/6 = 5/6


  1. Method of Least Common Denominator

This method is used when subtracting two or more fractions. The knowledge of the Least Common Multiple (LCM) is needed in this method. The LCM of the denominators is determined and used in the calculations. To find the LCM of the denominators, follow the steps below;

List the multiples of each denominator

Select the multiples that are common – appearing for all the denominators

Choose the smallest among the common list of the multiples. This is the LCM

After getting the LCM, divide the LCM by each denominator and multiply the result by the numerator of each fraction separately before you proceed to simply subtract the ‘new’ numerators and using the LCM as the denominator. Simplify the resultant fraction.

To clearly illustrate the long theory, we will need an example;

Question; 1/2 – 1/4 – 1/5

List the multiples of each denominator

2 = 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22…

4 = 4, 8, 12, 16, 20, 24, 28, 32, 36, 40…

5 = 5, 10, 15, 20, 25, 30, 35, 40…

Select the common multiples


Choose the smallest = 20. This is the LCM.

[(1*10) – (1*5) – (1*4)]/20 = 9/20.