Adding & Subtracting Fractions
How do I add or subtract two fractions?
- Addition and subtraction are treated in exactly the same way when dealing with fractions
- Find the lowest common denominator
- Write each fraction as an equivalent fraction over this denominator
- Add (or subtract) the numerators and write this over a single denominator
- do not add the denominators
- Simplify the fraction by cancelling common factors
What do I do if any of the fractions are mixed numbers?
- Convert any mixed numbers into top heavy fractions
- Then add or subtract the fractions as usual
- Remember to convert the answer back to a mixed number if required
Worked example
(a) Find .
Find the lowest common denominator of 3 and 5
15 is the smallest number that can be divided by both 3 and 5
15 is the smallest number that can be divided by both 3 and 5
The lowest common denominator is 15
Write both fractions as equivalent fractions with a common denominator of 15
Multiply the numerator and denominator of the first fraction by 5
Multiply the numerator and denominator of the second fraction by 3
Multiply the numerator and denominator of the first fraction by 5
Multiply the numerator and denominator of the second fraction by 3
Add the numerators and write over a single denominator
There are no common factors that will cancel
(b) Find giving your answer as a mixed number.
Change the mixed number into a top heavy fraction
Multiply the denominator by the whole part and add to the numerator
Write this value over the denominator
Find the lowest common denominator of 4 and 8
8 is the smallest number that by divided by both 4 and 8
8 is the smallest number that by divided by both 4 and 8
The lowest common denominator is 8
Write both fractions as equivalent fractions over 8
Multiply the numerator and denominator of the first fraction by 2
Multiply the numerator and denominator of the first fraction by 2
Subtract the numerators and write over a single denominator
Change into a mixed number
Divide 25 by 8 to get 3 remainder 1
Divide 25 by 8 to get 3 remainder 1