A radioactive sample has a half-life of 8.0 days. If the initial activity is 6400 decays per second, what is the activity after 24 days?
Show the answer and explanation
Correct answer
A. 800 decays/s
Principle or equation
Radioactive decay follows A = A_0 (1/2)^(t/T_1/2).
Why this answer is correct
24 days is 3 half-lives. A = 6400 × (1/2)^3 = 6400 / 8 = 800 decays/s.
Example
If half-life is 6 h and initial mass is 80 mg, after 18 h (3 half-lives) remaining is 10 mg.
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