Single correct option
A radioactive sample has a half-life of 10 days. If the sample initially has 80 g of the radioactive substance, the amount left after 30 days will be
- A. 20 g
- B. 26.7 g
- C. 40 g
- D. 10 g
Show answer and solution
Answer: (D) 10 g
30 days is 3 half-lives. After each half-life the amount halves: 80 to 40 to 20 to 10 g. The trap is to divide 80 by the number of half-lives, or to think only 1 half-life has elapsed. Use N = N0 (1/2)^(t/T) = 80 x 1/8 = 10 g.
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