document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=978662>Question 978662</A>: <I><SPAN STYLE=\"word-break: break-all;\"> 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) = A0e^-0.087t, where t is the time in days? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #600038 by </B><A HREF=/tutors/aboutme.mpl?userid=Boreal><B>Boreal(15235)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=Boreal><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/cgi-bin/show-face.mpl?user=Boreal><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=600038\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> A (t) = A0e^-0.087t<BR>\n" );
document.write( "A(t)/Ao= 0.25, because A(t) is the new level, and that is 25% of the original level.\r<BR>\n" );
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document.write( "0.25=e^(-0.087t)\r<BR>\n" );
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document.write( "take ln of both sides<BR>\n" );
document.write( "-1.38629= -0.087t\r<BR>\n" );
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document.write( "t=15.93 days\r<BR>\n" );
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document.write( "e^(-0.087*16)=0.2485.  This makes sense.\r<BR>\n" );
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