document.write( "Question 1204529: Radium-221 has a half-life of 30 seconds. How long will it take for 81% of a sample to decay? \n" ); document.write( "
Algebra.Com's Answer #840827 by ikleyn(52835)\"\" \"About 
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\n" ); document.write( "Radium-221 has a half-life of 30 seconds. How long will it take for 81% of a sample to decay?
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document.write( "Radium-221 has a half-life of 30 seconds.\r\n" );
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document.write( "Mathematically, it mean  M(t) = \"M%5B0%5D%2A%281%2F2%29%5E%28t%2F30%29\",\r\n" );
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document.write( "where M(t) is the current remaining mass, \"M%5B0%5D\" is the initial mass, t is the time, in seconds.\r\n" );
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document.write( "They want you find the time when 81% of the initial uranium isotope will decay;\r\n" );
document.write( "hence, 19%, or 0.19 of the initial mass remains.\r\n" );
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document.write( "For it, you write the decay equation in terms of half-life\r\n" );
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document.write( "    0.19 = \"%281%2F2%29%2A%28t%2F30%29\".\r\n" );
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document.write( "Take logarithm base 10 of both sides\r\n" );
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document.write( "    log(0.19) = \"%28t%2F30%29%2Alog%28%282%29%29\",\r\n" );
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document.write( "    t = \"-%2830%2Alog%28%280.19%29%29%29%2Flog%28%282%29%29\" = 71.9  seconds  (rounded).    ANSWER\r\n" );
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\n" ); document.write( "\n" ); document.write( "On radioactive decay,  see the lesson\r
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\n" ); document.write( "\n" ); document.write( "You will find many similar  (and different)  solved problems there.\r
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\n" ); document.write( "\n" ); document.write( "Use this lesson as your handbook,  textbook,  guide,  tutorials, and  (free of charge)  home teacher.\r
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