document.write( "Question 98507: An herbicide loses one-half its toxic effect each week. After how many weeks will the herbicide have less than 10 percent of its original potency? \n" ); document.write( "
Algebra.Com's Answer #71674 by stanbon(75887)\"\" \"About 
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An herbicide loses one-half its toxic effect each week. After how many weeks
\n" ); document.write( "will the herbicide have less than 10 percent of its original potency?
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\n" ); document.write( "let Po be the original potency.
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\n" ); document.write( "If you figure the answer in days
\n" ); document.write( "Po(1/2)^(t/7) < 0.10Po
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\n" ); document.write( "Divide both sides by Po to get:
\n" ); document.write( "(1/2)^(t/7) < 0.10
\n" ); document.write( "Take the log to get:
\n" ); document.write( "(t/2)log(1/2)\n" ); document.write( "Since log(1/2) is negative you get:
\n" ); document.write( "(t/7) > [log0.10]/[log(1/2)
\n" ); document.write( "t/7 > 3.32
\n" ); document.write( "t > 23.25 days
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\n" ); document.write( "Starting with the 24th day, the percentage will be < 10% of the original amount.
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\n" ); document.write( "Cheers,
\n" ); document.write( "Stan H.
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