Manganese-52 has a half-life of 6 days. how many days would a scientist have to wait for the radioactivity to be 12.5% the starting amount?
a.6
b.12
c.18
d.24

The answer is c. 18

It can be calculated using the equation:
= decimal amount remaining, where n i number of half-lives.

Half-life is 6 days. Let's calculate number of half-lives.


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Now we know that number of half-lives is 3.
Number of half-lives is quotient of total time elapsed and length of half-life.
So, total time elapsed is a product of length of half-life (6 days) and number of half-lives (3). Since 6 × 3 = 18, then a scientist will have to wait 18 days for the radioactivity to be 12.5% the starting amount.