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CIE iGCSE Co-Ordinated Science P5.2.4 Half-life Exam Style Questions Paper 2

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Question

A radioactive isotope has a half-life of 18 years. A sample contains 80 million atoms of this isotope.

How long does it take for the number of atoms of this isotope in the sample to decrease to 10 million?

A. 2.25 years
B. 6.0 years
C. 54 years
D. 180 years

▶️ Answer/Explanation
Each half-life halves the number of atoms: (80 \to 40 \to 20 \to 10) million, which takes 3 half-lives.
Total time = (3 \times 18 = 54) years.
✅ Answer: (C)
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