A radioactive sample has 320 g of a nuclide. How much remains after 4 half-lives?

Answer and explanation

Correct answer: B. 20 g

  1. Step 1: After each half-life the mass is halved, so the remaining mass is N0 / 2^n.
  2. Step 2: 320 / 2^4 = 20 g.
Answer
20 g.
Why not the others: '80 g' divides by the number of half-lives instead of 2^n; '40 g' divides by 2n instead of 2^n; '10 g' counts one half-life too many.

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