Half-life from a drop in activity
A sample's activity drops from 800 to 100 counts per minute over 6 hours. What is its half-life?
Answer B: explanation for every choice
A
This divides 6 h by 8, treating the decay as linear.
B
Correct. 800 → 400 → 200 → 100 is 3 half-lives in 6 h, so each half-life is 2 h.
C
This assumes 2 half-lives.
D
8 is the overall ratio of activities, not a time.
TakeawayCount the halvings: 2ⁿ = starting activity ÷ final activity.
Equation usedRadioactive decay
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