document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1005725>Question 1005725</A>: <I><SPAN STYLE=\"word-break: break-all;\"> After 3 days a sample of Radon-222 has decayed to 50% of its original amount. <BR>\n" );
document.write( "1. What is the half life?\r<BR>\n" );
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document.write( "2. How long will it take the sample to decay to 30% of its original amount? \r<BR>\n" );
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document.write( "Please help me !! </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #621880 by </B><A HREF=/tutors/aboutme.mpl?userid=fractalier><B>fractalier(6550)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=fractalier><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=fractalier><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=621880\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> The half-life IS three days, since half of it is gone in that time.<BR>\n" );
document.write( "From there we use<BR>\n" );
document.write( "A(t) = Ao * (1/2)^(t/3)<BR>\n" );
document.write( ".3Ao = Ao * (1/2)^(t/3)<BR>\n" );
document.write( ".3 = (1/2)^(t/3)<BR>\n" );
document.write( ".3 = 2^(-t/3)<BR>\n" );
document.write( "ln .3 = (-t/3) ln 2<BR>\n" );
document.write( "ln .3 / ln 2 = -t/3<BR>\n" );
document.write( "t = -3ln .3 / ln 2 = 5.21 days<BR>\n" );
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