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?

Each half-life reduces the radioactivity by half. After 6 days it is 50%, after 12 days it is 25%, and after 18 days it is 12.5% of the starting amount. Therefore, the scientist must wait 18 days, choice C.

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