Question

A radioactive substance has a half-life of 5 days. If its initial mass is 80 g, the mass remaining after 15 days is:

• A. 40 g
• B. 20 g
• C. 10 g
• D. 5 g

Hint:

For small integers of half-lives (like $1, 2, 3$, or $4$), the step-by-step halving method is usually faster and less prone to calculation errors than using the formula.
Simply divide the mass by $2$ successively for each half-life: $80 \rightarrow 40 \rightarrow 20 \rightarrow 10$.

Answer 1

The Correct Option is C

Step 1: Half-lives.
$n=15/5=3$. Remaining $= 80\times(1/2)^3=80/8$
\[ \boxed{10 \text{ g}} \]