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Answer: 4. 10 minutes

Key idea: first order: \(k=\frac{0.693}{t_{1/2}}\) and \(t=\frac{2.303}{k}\log\frac{a}{a-x}\).

  1. \(k=0.693\ \mathrm{min^{-1}}\).
  2. 99.9% complete: \(\frac{a}{a-x}=1000\), \(\log=3\).
  3. \(t=\frac{2.303\times3}{0.693}\approx9.97\) min.

Closest is 10 min: option (4). (Equivalent to about 10 half-lives, since \(2^{10}\approx1000\).)

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