Since the multiples of 4 include 4, 8, 12, 16, 20 and the multiples of 5 include 5, 10, 15, and 20, 20 is the lowest number they have in common. If the denominators are already the same, you can skip straight to subtracting the numerators.
For example, multiply 1/4 by 5 to get a denominator of 20. You’ll also need to multiply the numerator by 5, so 1/4 becomes 5/20.
For example, if you’ve adjusted 1/4 to become 5/20, multiply the 1/5 by 4 to get 4/20. The original problem 1/4 - 1/5 becomes 5/20 - 4/20.
Remember not to subtract the denominators as well. For example, 5/20 - 4/20 = 1/20.
For example, 2 3/4 - 1 1/7 will become 11/4 - 8/7.
Since the multiples of 4 include 4, 8, 12, 16, 20, 24, 28 and the multiples of 7 include 7, 14, 21, and 28, 28 is the lowest number they have in common.
For example, to make the denominator for 11/4 become 28, multiply the fraction by 7. The fraction will become 77/28.
For example, if you’ve adjusted 11/4 to become 77/28, multiply the 8/7 by 4 to get 32/28. The problem 11/4 - 8/7 becomes 77/28 - 32/28.
For example, 77/28 - 32/28 = 45/28.
For example, 45/28 becomes 1 17/28 because 28 goes into 45 1 time and has 17 parts of 28 left over.
