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Syllabus Edition
First teaching 2021
Last exams 2024
Adding & Subtracting Fractions (CIE IGCSE Maths: Core)
Revision Note
Adding & Subtracting Fractions
Dealing with mixed numbers
- Always turn mixed numbers into top heavy fractions before adding or subtracting
Adding & subtracting
- Adding and subtracting are treated in exactly the same way:
- Find the lowest common denominator (the smallest whole number that each denominator divides)
- Write each fraction as an equivalent fraction over this denominator (by multiplying top-and-bottom by the same amount)
- Add (or subtract) the numerators and write this over a single lowest common denominator
- do not add the denominators
- Check for any cancellation (or if asked to turn top heavy fractions back into mixed numbers)
Worked example
(a) Find
Find the lowest common denominator of 3 and 5
15 is the smallest number that divides both 3 and 5
the lowest common denominator is 15
Write both fractions as equivalent fractions over 15 (by multiplying top and bottom by the same amount)
Add the numerators and write over a single denominator
There is no cancellation
(b) Find giving your answer as a mixed number
Change the mixed number into a top heavy fraction (by multiplying the denominator, 4, by the whole number, 3, then adding the numerator, 3)
To find first find the lowest common denominator of 4 and 8
8 is the smallest number that divides both 4 and 8
the lowest common denominator is 8
Write both fractions as equivalent fractions over 8 (by multiplying top and bottom by the same amount)
Subtract the numerators and write over a single denominator
Change into a mixed number (by dividing 25 by 8 to get 3 remainder 1)
There is no more cancellation
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