At a toy shop, there were 48 robot kits and 216 board-game boxes. The shop received an equal number of robot kits and board-game boxes. After the new stock was added, the ratio of robot kits to board-game boxes became 1 : 3. How many robot kits were added?
Need a hint?
Adding equal numbers does not change the difference between the two quantities.
Worked answer
- The difference stayed the same: 216 − 48 = 168.
- In the ratio 1 : 3, the difference of 2 parts was 168, so 1 part was 168 ÷ 2 = 84.
- The number of robot kits added was 84 − 48 = 36.
Answer: 36 robot kits
The mistake this question tests
Habit
Stopped at one part in ratio
Taking 216 ÷ 3 = 72 as the final number of robot kits ignores that new stock was added to both quantities. The ratio applies to the final quantities.
How to fix it at home →Wrong answers children often give:
- 24 Stopped at one part in ratio
- 72 Stopped at one part in ratio
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See the answer →Practise PSLE ratio until the mistakes stop.
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