Radioactive decay with N = N₀(½)^(t/t½): leave any one of N₀, N, t or the half-life blank and it is calculated from the other three.
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From the article Half-Life Formula — Radioactive Decay Calculations.
A 100 g sample of a nuclide has t½ = 5.0 days. How much remains after 20 days?
n = t ÷ t½ = 20 ÷ 5.0 = 4 half-lives
N = 100 × (1/2)⁴ = 100 ÷ 16 = 6.25 g
and 100 − 6.25 = 93.75 g has decayed. Whenever n is a whole number, never touch the exponential — the halving form is faster and safer.
Worked in full in Half-Life Formula — Radioactive Decay Calculations.
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