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The half-life of a substance is 40.240.2 million years. If a certain amount is only considered safe when its radioactivity has dropped to \newline6.25%6.25\% of the original level, approximately how much time must the substance be stored securely to be safe?\newline(A)160.8(A)160.8 billion years\newline(B)160.8(B)160.8 million years\newline(C)201(C)201 million years\newline(D)201(D)201 billion years

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Q. The half-life of a substance is 40.240.2 million years. If a certain amount is only considered safe when its radioactivity has dropped to \newline6.25%6.25\% of the original level, approximately how much time must the substance be stored securely to be safe?\newline(A)160.8(A)160.8 billion years\newline(B)160.8(B)160.8 million years\newline(C)201(C)201 million years\newline(D)201(D)201 billion years
  1. Determine Half-Lives: We need to determine how many half-lives it takes for a substance to reach 6.25%6.25\% of its original radioactivity level.\newline6.25%6.25\% is equivalent to 6.25100\frac{6.25}{100}, which simplifies to 116\frac{1}{16}. This means the substance needs to go through enough half-lives to be 116\frac{1}{16}th of its original amount.
  2. Express as Power: To find out how many half-lives it takes to reach 116\frac{1}{16}th of the original amount, we can express 116\frac{1}{16} as a power of 12\frac{1}{2}, since each half-life reduces the substance by half.116\frac{1}{16} is the same as (12)4(\frac{1}{2})^4, because (12)4=116(\frac{1}{2})^4 = \frac{1}{16}. This means it takes 44 half-lives to reach 6.25%6.25\% of the original radioactivity level.
  3. Calculate Total Time: Now that we know it takes 44 half-lives to reach 6.25%6.25\% of the original radioactivity, we can calculate the total time by multiplying the number of half-lives by the duration of one half-life.\newlineThe half-life is given as 40.240.2 million years, so we multiply 44 half-lives by 40.240.2 million years per half-life.\newline4×40.24 \times 40.2 million years =160.8= 160.8 million years.

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