0580/42 May/Jun 2026, Question 12: Exponential decay
Cambridge IGCSE Mathematics 0580 · Paper 42 (Extended) · Calculator · 3 marks
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The question, in short
A substance weighs 150 g on 10 January and loses 5% of its mass each day. On what date is its mass first recorded below 96 g?
Summarised in our own words. Use the official paper for the exact wording and any diagrams.
Solution · 3 marks
- 1Mass after n days = 150 × 0.95ⁿ
- 2n = 8: 150 × 0.95⁸ = 99.5… g (not yet below 96)
- 3n = 9: 150 × 0.95⁹ = 94.5… g < 96
- 410 January + 9 days = 19 January
Answer19 January
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