Adding & Subtracting Fractions (Edexcel GCSE Maths: Foundation)

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Naomi C

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Naomi C

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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 2 over 3 plus 1 fifth.

Find the lowest common denominator of 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
 
fraction numerator 2 cross times 5 over denominator 3 cross times 5 end fraction plus fraction numerator 1 cross times 3 over denominator 5 cross times 3 end fraction equals 10 over 15 plus 3 over 15 
 
Add the numerators and write over a single denominator
 

fraction numerator 10 plus 3 over denominator 15 end fraction equals 13 over 15 

There are no common factors that will cancel

bold 13 over bold 15

 

(b)     Find 3 3 over 4 minus 5 over 8 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
 

fraction numerator 4 cross times 3 plus 3 over denominator 4 end fraction equals 15 over 4 

Find the lowest common denominator of 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
 

fraction numerator 15 cross times 2 over denominator 4 cross times 2 end fraction minus 5 over 8 equals 30 over 8 minus 5 over 8

Subtract the numerators and write over a single denominator
 

fraction numerator 30 minus 5 over denominator 8 end fraction equals 25 over 8

Change into a mixed number
Divide 25 by 8 to get 3 remainder 1

bold 3 bold 1 over bold 8

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Naomi C

Author: Naomi C

Expertise: Maths

Naomi graduated from Durham University in 2007 with a Masters degree in Civil Engineering. She has taught Mathematics in the UK, Malaysia and Switzerland covering GCSE, IGCSE, A-Level and IB. She particularly enjoys applying Mathematics to real life and endeavours to bring creativity to the content she creates.