0580/41 Oct/Nov 2025, Question 19: Exponential decay
Cambridge IGCSE Mathematics 0580 · Paper 41 (Extended) · Calculator · 3 marks
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The question, in short
20 g of a substance loses 10% of its mass each day. Find how many whole days it takes for the mass to first fall below 1 g.
Summarised in our own words. Use the official paper for the exact wording and any diagrams.
Solution · 3 marks
- 1Mass after n days = 20 × 0.9ⁿ
- 2n = 28: 20 × 0.9²⁸ = 1.046… g (not less than 1)
- 3n = 29: 20 × 0.9²⁹ = 0.942… g (less than 1)
- 429 days
Answer29
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