document.write( "Question 1128536: After the release of radioactive material into the atmosphere from a nuclear power plant in a country in 1981
\n" ); document.write( ",
\n" ); document.write( "the hay in that country was contaminated by a radioactive isotope (half-life 7
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\n" ); document.write( "days). If it is safe to feed the hay to cows when 15
\n" ); document.write( "%
\n" ); document.write( "of the radioactive isotope remains, how long did the farmers need to wait to use this hay? \n" ); document.write( "

Algebra.Com's Answer #745021 by ankor@dixie-net.com(22740) $\"\"$ $\"About$

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After the release of radioactive material into the atmosphere from a nuclear power plant in a country in 1981, the hay in that country was contaminated by a radioactive isotope (half-life 7 days).
\n" ); document.write( " If it is safe to feed the hay to cows when 15% of the radioactive isotope remains, how long did the farmers need to wait to use this hay?
\n" ); document.write( ":
\n" ); document.write( "Using the radio-active decay formula; $\"A+=+Ao%2A2%5E%28-t%2Fh%29\"$ , where
\n" ); document.write( "A = resulting amt after t time
\n" ); document.write( "Ao = initial amt
\n" ); document.write( "t = time in days
\n" ); document.write( "h = half-life in days
\n" ); document.write( ":
\n" ); document.write( "let A=1, the Ao= .15
\n" ); document.write( ":
\n" ); document.write( " $\"1%2A2%5E%28%28-t%2F7%29%29+=+.15\"$
\n" ); document.write( " $\"-t%2F7\"$ = $\"ln%28.15%29%2Fln%282%29\"$
\n" ); document.write( " $\"-t%2F7\"$ = -2.737
\n" ); document.write( "t = -7 * -2.737
\n" ); document.write( "t = +19.158 days, say cow can start eating it in 20 days safely \n" ); document.write( "

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