A sample has passed through three half-lives. Why is the amount left one-eighth, not zero?
A half-life removes the same fraction, not the same amount.
A half-life removes the same fraction, not the same amount. Starting with N_0: after one half-life N_0/2 remains, after two N_0/4, and after three N_0/8. Exponential decay approaches zero but the ideal model never reaches it at a finite time. Use N/N_0 = (1/2)^n when an integer number n of half-lives has passed.
Key point
After n half-lives, fraction remaining is 2^(-n).
Common mistake
subtracting N_0/2 during every interval.
Memory tip
halve what remains, not what you started with.
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