A box contains red and blue beads. Ali took out 30 red beads and 25% of the blue beads. After that, the ratio of the number of red beads to the number of blue beads in the box was 3 : 7.
- What fraction of the blue beads did Ali take out? Give your answer in simplest form.
- At first, there were 160 more blue beads than red beads in the box. How many beads were left in the box in the end?
Need a hint?
Find the number of beads left of each colour first, using the ratio 3 : 7. Then work backwards to the number at first.
Worked answer
- (a) 25% = 14, so Ali took out 14 of the blue beads.
- (b) Red left : blue left = 3 : 7 = 9 : 21. Let red left = 9 parts and blue left = 21 parts.
- (b) Blue left is 34 of the blue at first, so blue at first = 21 ÷ 3 × 4 = 28 parts.
- (b) Red at first = 9 parts + 30.
- (b) Blue at first − red at first = 160, so 28 parts − 9 parts − 30 = 160, and 19 parts = 190.
- (b) 1 part = 10. Red left = 90 and blue left = 210.
- (b) Beads left in the end = 90 + 210 = 300.
The mistake this question tests
Stopped at one part in ratio
Many pupils treat the 25% as a fraction of the beads left, not of the blue beads at first, or they stop after finding the blue beads left (210) and forget to add the red beads left.
How to fix it at home →More PSLE ratio questions
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See the answer →Practise PSLE ratio until the mistakes stop.
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