ACT Preparation / Unit 12 ACT 1121 short lesson, about 20 seconds
Half-Life and Doubling Time
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The idea
A quantity with half-life h is A = A₀(1/2)^(t/h): it halves every h units of time. A quantity with doubling time d is A = A₀ · 2^(t/d).
In practice
80 grams with a half-life of 5 days leaves 80(1/2)³ = 10 grams after 15 days.
Where people slip
Count half-lives first: 15 days is three half-lives, not fifteen.
Your place on the trail
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