document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=202639>Question 202639</A>: <I><SPAN STYLE=\"word-break: break-all;\"> The half-life of a substance is the time it takes for half of the substance to remain after natural decay.  Radioactive water (tritium) has a half-life of 12.6 years. How long will it take for 85% of a sample to decay? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #152862 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=152862\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> The half-life of a substance is the time it takes for half of the substance<BR>\n" );
document.write( " to remain after natural decay. Radioactive water (tritium) has a half-life<BR>\n" );
document.write( " of 12.6 years. How long will it take for 85% of a sample to decay?<BR>\n" );
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document.write( "The half life formula:<BR>\n" );
document.write( "A = Ao<BR>\n" );
document.write( "where:<BR>\n" );
document.write( "A = the resulting amt after t (yrs in this case)<BR>\n" );
document.write( "Ao = initial amt<BR>\n" );
document.write( "t = time (yrs)<BR>\n" );
document.write( "h = half-life of substance (yrs)<BR>\n" );
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document.write( "Let initial amt: Ao = 1, then find A: 1.0 - .85 = .15<BR>\n" );
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document.write( "1*2^(-t/12.6) = .15<BR>\n" );
document.write( "Find the log of both sides<BR>\n" );
document.write( ".301<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-t%2F12.6 ALIGN=MIDDLE  ALT=\"-t%2F12.6\"   DISABLED_style= \"max-width:90%; height:auto;\" > = -.8239<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-.301t%2F12.6 ALIGN=MIDDLE  ALT=\"-.301t%2F12.6\"   DISABLED_style= \"max-width:90%; height:auto;\" > = -.8239<BR>\n" );
document.write( "Multiply both sides by 12.6<BR>\n" );
document.write( "-.301t = -.8239 * 12.6<BR>\n" );
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document.write( "-.301t = -10.381<BR>\n" );
document.write( "t = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=%28-10.381%29%2F%28-.301%29 ALIGN=MIDDLE  ALT=\"%28-10.381%29%2F%28-.301%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
document.write( "t = 34.49 yrs<BR>\n" );
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document.write( "Check solution on a calc: enter 2^(-34.49/12.6) = .1499 ~ .15<BR>\n" );
document.write( " </SPAN>\n" );
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