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Answer: 3. 60

Use \(N=N_0\left(\tfrac12\right)^{t/T}\) and compare the remaining fractions.

  • 40% decayed means 60% remains; 85% decayed means 15% remains.
  • Ratio of remaining amounts: \(\dfrac{60}{15}=4=2^2\).

The amount must be halved twice, which takes \(2\times30=60\) minutes. Option (3).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report
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