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Toy cars and robots — after some robots are sold
A toy store had 280 more toy cars than toy robots. After 140 toy robots were sold, the ratio of the number of toy robots to the number of toy cars was 2 : 7. How many toy cars were there?
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The pattern behind it: Constant part
The ratio changes, but one person's amount stays the same.
In this one. The cars are never touched, so selling 140 robots widens the 280 gap by exactly 140 — and that new gap of 420 is what the ratio's 7 − 2 = 5 units measure.
How to spot it
A ratio changes and the question tells you only one person gained or lost an amount, while the other kept what they had.
Common mistake
Children keep the same number of shares for the person whose amount did not change. The amount stays the same, but the size of a share can differ between the two ratios.
Try one like it
Challenging Ali and Ben's savings — the ratio shifts The ratio of Ali's savings to Ben's savings was 3 : 5. After Ali saved another $24, the ratio became 9 : 10. How much did Ali have…The 33 problem patterns
All 33 patterns, one to a page — the 19 behind Primary 5 and 6 word problems, and the skills the younger years are built on. The bar model, how to spot it in the question, and the mistake children make. A booklet you can print. Free.
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