Ever wondered how much zobo is left in the bottle?
Ngozi had 34 of a bottle of zobo and drank 12 of a bottle at lunch. How much is left? Quarters and halves are different sizes, so we need one small trick before we can take away.
Revision notes
Take fractions away — with the same or different denominators, from whole numbers and with mixed numbers.
Ngozi had 34 of a bottle of zobo and drank 12 of a bottle at lunch. How much is left? Quarters and halves are different sizes, so we need one small trick before we can take away.
If the denominators are the same, the pieces are the same size. Keep the denominator and subtract the numerators.
79 − 49 = 39 = 13. Seven ninths take away four ninths leaves three ninths.
Never subtract the denominators — taking pieces away does not change the size of each piece.
🔑 Key idea: Same denominator → keep it, subtract the numerators, then simplify.
For 34 − 12, the pieces are different sizes. Use the LCM of the denominators as a common denominator: the LCM of 4 and 2 is 4.
12 = 24, so 34 − 24 = 14. Ngozi has 14 of a bottle left.
Whatever you multiply the bottom by, multiply the top by the same number.
🔑 Key idea: Change both fractions to the LCM of the denominators first.
To work out 3 − 25, write 3 as fifths: 3 = 155. Then 155 − 25 = 135 = 235.
For mixed numbers like 314 − 123, change both to improper fractions first: 134 − 53.
Then use a common denominator (12): 3912 − 2012 = 1912 = 1712.
🔑 Key idea: Mixed numbers → improper fractions → common denominator → subtract.
Work out 78 − 13.
Step 1: Find a common denominator
LCM of 8 and 3 = 24
Why? We can only take away pieces of the same size.
Step 2: Rewrite both fractions
78 = 2124 (× 3 top and bottom)
13 = 824 (× 8 top and bottom)
Step 3: Subtract the numerators
2124 − 824 = 1324
Answer: 1324
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