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Remaining = initial × (1/2)^(t/t½), or equivalently initial × e^(−λt) where λ = ln(2)/t½. After n half-lives, (1/2)^n of the original remains. Carbon-14 has a 5,730-year half-life (used in radiocarbon dating). Ibuprofen's half-life is ~2 hours (5 doses ≈ 97% eliminated). Half-life is constant regardless of starting quantity—a key property of first-order processes.
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