document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=61670>Question 61670</A>: <I><SPAN STYLE=\"word-break: break-all;\"> The half- life of a radioactive substance is one hundred ninety-four days. How many days will it take for eighty percent of the substance to decay?\r<BR>\n" );
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document.write( "I used the half life decay formula.<BR>\n" );
document.write( "my formula for the problem read...<BR>\n" );
document.write( "0.8Nsubscript0=Nsubscript0*(1/2)superscript(t/194)<BR>\n" );
document.write( "the result was about 62.5,<BR>\n" );
document.write( "is this correct.\r<BR>\n" );
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document.write( "=] </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #42455 by </B><A HREF=/tutors/aboutme.mpl?userid=ankor@dixie-net.com><B>ankor@dixie-net.com(22740)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=ankor@dixie-net.com><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=ankor@dixie-net.com><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=42455\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\">  The half- life of a radioactive substance is one hundred ninety-four days. How many days will it take for eighty percent of the substance to decay? <BR>\n" );
document.write( "I used the half life decay formula.<BR>\n" );
document.write( "my formula for the problem read...<BR>\n" );
document.write( "0.8Nsubscript0=Nsubscript0*(1/2)superscript(t/194)<BR>\n" );
document.write( "the result was about 62.5,<BR>\n" );
document.write( "is this correct. <BR>\n" );
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document.write( "This equation tells you what is left after a period of time, If 80% decayed wouldn't the remainder be .2? \r<BR>\n" );
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document.write( "I would write it:<BR>\n" );
document.write( "A = Ao*.5^(t/h)<BR>\n" );
document.write( "Where:<BR>\n" );
document.write( "A = .2<BR>\n" );
document.write( "Ao = original amt, 1.0 in this case<BR>\n" );
document.write( "t = time in days<BR>\n" );
document.write( "h = half life in days<BR>\n" );
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document.write( ".2 = 1 * .5^(t/194)<BR>\n" );
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document.write( "ln(.2) = (t/194)*ln(.5)<BR>\n" );
document.write( "-1.6094 = (t/194)* -.6931<BR>\n" );
document.write( "Which arranges to:<BR>\n" );
document.write( "t/194 = -1.6094/-6931<BR>\n" );
document.write( "t = 194 * 2.3219<BR>\n" );
document.write( "t = 450.45 days for 80% to decay<BR>\n" );
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document.write( "Check it:<BR>\n" );
document.write( "A = 1*.5^(450.24/194) = .20001<BR>\n" );
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document.write( "Did this help? </SPAN>\n" );
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