In more detail
The remaining amount is N₀ × (½)^(t / T), where T is the half-life; the decay constant is λ = ln 2 / T. Each half-life removes half of whatever is left, so after one, two, three, four and five half-lives 50, 25, 12.5, 6.25 and 3.125% remain, and the amount never reaches exactly zero. Half-lives range from fractions of a second to billions of years: carbon-14 is about 5,730 years and iodine-131 about 8 days. Caffeine in the body is often quoted at around five hours, though it varies widely between people.
Why it matters
In medicine a drug given regularly reaches a steady level, and is mostly cleared after stopping, in about four to five half-lives, which shapes dosing intervals. Radiocarbon dating reads age from how much carbon-14 is left. A common mistake is to think two half-lives remove everything; they remove three quarters.
Example
80 g, half-life 5 days, after 15 days: 80 × (½)^(15/5) = 10 g
200 mg caffeine at 08:00, T = 5 h, 18:00: 200 × (½)^2 = 50 mg
After five half-lives: (½)^5 = 3.125 % left
Carbon-14 down to 10 %: 5,730 × log₂(10) ≈ 19,035 years