- The half-life of sodium-24 is 15.0 hours.
- The sample was stored for 45.0 hours.
- To find the number of half-lives, divide the total time by the half-life:
\[
\text{Number of half-lives} = \frac{45.0 \text{ hours}}{15.0 \text{ hours/half-life}} = 3 \text{
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