If a substance has a half-life of 3 days, what fraction remains after 3 days?

If a substance has a half-life of 3 days, what fraction remains after 3 days?

• A. 1/2
• B. 1/3
• C. 1/4
• D. 1

Answer: (A)

Half-life is the time it takes for a substance to fall to half of its current amount. After one half-life, the remaining fraction is 1/2. Since the given half-life is 3 days, exactly 3 days pass for one half-life, so 1/2 remains. The other fractions would correspond to different timing: 1/3 isn’t tied to a single half-life, 1/4 would require two half-lives (6 days), and 1 would mean no decay.

Half-life is the time it takes for a substance to fall to half of its current amount. After one half-life, the remaining fraction is 1/2. Since the given half-life is 3 days, exactly 3 days pass for one half-life, so 1/2 remains. The other fractions would correspond to different timing: 1/3 isn’t tied to a single half-life, 1/4 would require two half-lives (6 days), and 1 would mean no decay.