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Tank X and Tank Y held water in the ratio 7 : 9.
Tank X and Tank Y held water in the ratio 7 : 9. More water was then poured into Tank X only, 4 litres worth, so the ratio became 4 : 5. How many litres did Tank X hold at first?
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The pattern behind it: Constant part
The ratio changes, but one person's amount stays the same.
In this one. Hold the unchanged tank at the same number of parts in both ratios — then the water poured in is the whole of the difference.
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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