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Leçon 9 · classe 7–9
Rule of three and real-life percentages
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The rule of three: you know 3 numbers, find the 4th through proportion. 4 kg of apples cost 8 €, what do 7 kg cost? First the price per kg (2 €), then ×7 = 14 €. Interest is percentage with money: 1,000 € at 4% per year = +40 €. Discounts are the same math: 80 € minus 25% = 80 × 0.75 = 60 €.
- ●Proportion: first "per one", then "per how many"
- ●Interest: capital × rate × time
- ●Inverse: 3 builders in 12 days → 6 builders in 6
💶 Compound interest — interest on interest: 100 € at 10% becomes 121 € in 2 years, not 120. Everything builds on it: from savings to loans.
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