document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=946030>Question 946030</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Radium-221 has a half-life of 30 sec. How long will it take for 91% of a sample to decay? (Round your answer to the nearest whole number.)<BR>\n" );
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document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #577103 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=577103\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\">  Radium-221 has a half-life of 30 sec. How long will it take for 91% of a sample to decay? <BR>\n" );
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document.write( "That leaves 9% remaining<BR>\n" );
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document.write( "The formula; A = Ao*2^(-t/h) where:<BR>\n" );
document.write( "A = remaining amt after t time<BR>\n" );
document.write( "Ao = initial amt (t=0)<BR>\n" );
document.write( "t = time of decay<BR>\n" );
document.write( "h = half-life of substance<BR>\n" );
document.write( "In this problem let Ao = 1, then A = .09, given half-life of 30 sec<BR>\n" );
document.write( "1*2^(-t/30) = .09<BR>\n" );
document.write( "log(2^(-t/30) = log(.09)<BR>\n" );
document.write( "log equiv of exponents<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-t%2F30 ALIGN=MIDDLE  ALT=\"-t%2F30\"   DISABLED_style= \"max-width:90%; height:auto;\" >*log(2) = log(.09)<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-t%2F30 ALIGN=MIDDLE  ALT=\"-t%2F30\"   DISABLED_style= \"max-width:90%; height:auto;\" > = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=log%28%28.09%29%29%2Flog%28%282%29%29 ALIGN=MIDDLE  ALT=\"log%28%28.09%29%29%2Flog%28%282%29%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
document.write( "use the calc to find the logs<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-t%2F30 ALIGN=MIDDLE  ALT=\"-t%2F30\"   DISABLED_style= \"max-width:90%; height:auto;\" > = -3.474<BR>\n" );
document.write( "multiply both sides by -30<BR>\n" );
document.write( "t = -30 * -3.474<BR>\n" );
document.write( "t = +104.2 ~ 104 seconds </SPAN>\n" );
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