Five weeks ago, the ratio of Aisyah's sticker count to Jun's sticker count was 3 : 7. Aisyah and Jun each collect 1 sticker every week. In 2 weeks' time, Aisyah's sticker count will be 47 of Jun's sticker count. How many stickers has Jun collected now?
- 16 stickers
- 21 stickers
- 26 stickers
- 28 stickers
Need a hint?
Let the counts from 5 weeks ago be 3x and 7x. Then track both counts to 2 weeks from now.
Worked answer
- Let one part of the ratio be x stickers.
- Five weeks ago, Aisyah had 3x stickers and Jun had 7x stickers.
- Now, their counts are 3x + 5 and 7x + 5.
- In 2 more weeks, the total increase from 5 weeks ago is 7, so the counts will be 3x + 7 and 7x + 7.
- Since Aisyah's future count is 47 of Jun's, 7(3x + 7) = 4(7x + 7).
- 21x + 49 = 28x + 28, so 7x = 21 and x = 3.
- Jun's current count is 7(3) + 5 = 26 stickers.
Answer: 26 stickers
The mistake this question tests
Habit
Stopped at one part in ratio
Using 3x and 7x as the present counts is a common mistake. Those are the counts from 5 weeks ago, so add 5 for the present and 7 for the future.
How to fix it at home →Wrong answers children often give:
- 16 stickers Calculation slip
- 21 stickers Stopped at one part in ratio
- 28 stickers Calculation slip
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See the answer →Structured · 4mA box contains red and blue beads
See the answer →MCQ · 2mIn a park, the ratio of the number of trees in Garden P to the number of trees in Garden Q is 3 : 5
See the answer →Practise PSLE ratio until the mistakes stop.
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