A community centre kept exercise books in Box R and Box S. Box R contained twice as many books as Box S. In Box R, 35 of the books had coloured covers. In Box S, 45 of the books had coloured covers. What fraction of all the books had coloured covers?
Need a hint?
Represent the two box sizes using their ratio before finding the coloured books in each box.
Worked answer
- Take Box S as 5 books; then Box R has 10 books.
- Coloured books: 10 × 35 = 6 and 5 × 45 = 4.
- Fraction coloured: (6 + 4) ÷ (10 + 5) = 23.
The mistake this question tests
Stopped at one part in ratio
Averaging 35 and 45 gives 710, but this wrongly gives equal weight to the two boxes. Box R contains twice as many books as Box S.
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See the answer →Practise PSLE fractions until the mistakes stop.
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