Subtracting Mixed Numbers with Different Denominators

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You can subtract mixed numbers only if they have the same denominator, so you must first change each of the two fractions to a common denominator.

Example 1 Suppose you have two mixed numbers 3 3/4 and 2 1/3 to be subtracted

3	3		2	1
	---	-		---	=		---
	4			3

First you must find common denominators.

Select the denominator of the second fraction (3) and multiply the numerator and denominator of the first fraction (3/4) by that number:

3		3		9
---	x	---	=	---
4		3		12

Select the denominator of the first fraction (4) and multiply the numerator and denominator of the second fraction (1/3) by that number:

1		4		4
---	x	---	=	---
3		4		12

These two fractions (9/12 and 4/12) have common denominators.

Subtract the two new fractions that have the common denominator. Do NOT subtract the denominators.

9		4		5
---	-	---	=	---
12		12		12

So your answer will be

3	9		2	4		1	5
	---	-		---	=		---
	12			12			12