document.write( "Question 599866: How long will it take any quantity of iodine 131 to decay to 25% of its initial amount, knowing that it decays according to the function defined by A(t)=A_0 e^(-.087t), where t is time in days? \n" ); document.write( "
Algebra.Com's Answer #379127 by nerdybill(7384)\"\" \"About 
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How long will it take any quantity of iodine 131 to decay to 25% of its initial amount, knowing that it decays according to the function defined by A(t)=A_0 e^(-.087t), where t is time in days?
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\n" ); document.write( "Let x = initial amount
\n" ); document.write( "then plugging in to the given equation we get:
\n" ); document.write( ".25x = xe^(-.087t)
\n" ); document.write( "dividing both sides by x:
\n" ); document.write( ".25 = e^(-.087t)
\n" ); document.write( "take ln of both sides:
\n" ); document.write( "ln(.25) = -.087t
\n" ); document.write( "ln(.25)/(-.087) = t
\n" ); document.write( "15.93 days = t\r
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