document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1005726>Question 1005726</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Radium-221 has a half life of 30 seconds, how long will it take for 72% of a sample to decay?\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 #621879 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=621879\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> We use the formula<BR>\n" );
document.write( "A(t) = Ao * (1/2)^(t/30)<BR>\n" );
document.write( ".28Ao = Ao * (1/2)^(t/30)<BR>\n" );
document.write( ".28 = (1/2)^(t/30)<BR>\n" );
document.write( "Now take ln and get<BR>\n" );
document.write( "ln .28 = (-t/30)ln 2<BR>\n" );
document.write( "t = -30 * ln(.28) / ln 2 = 55.1 seconds </SPAN>\n" );
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