The half-life of a substance is 40.2 million years. If a certain amount is only considered safe when its radioactivity has dropped to 6.25% of the original level, approximately how much time must the substance be stored securely to be safe?(A)160.8 billion years(B)160.8 million years(C)201 million years(D)201 billion years
Q. The half-life of a substance is 40.2 million years. If a certain amount is only considered safe when its radioactivity has dropped to 6.25% of the original level, approximately how much time must the substance be stored securely to be safe?(A)160.8 billion years(B)160.8 million years(C)201 million years(D)201 billion years
Determine Half-Lives: We need to determine how many half-lives it takes for a substance to reach 6.25% of its original radioactivity level.6.25% is equivalent to 1006.25, which simplifies to 161. This means the substance needs to go through enough half-lives to be 161th of its original amount.
Express as Power: To find out how many half-lives it takes to reach 161th of the original amount, we can express 161 as a power of 21, since each half-life reduces the substance by half.161 is the same as (21)4, because (21)4=161. This means it takes 4 half-lives to reach 6.25% of the original radioactivity level.
Calculate Total Time: Now that we know it takes 4 half-lives to reach 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.The half-life is given as 40.2 million years, so we multiply 4 half-lives by 40.2 million years per half-life.4×40.2 million years =160.8 million years.
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