document.write( "Question 165711This question is from textbook
\n" ); document.write( ": The level of thorium in a sample decreases by a factor of one half every 4.2 million years. A meteorite is discovered to have only 7.6% of its orginial thorium remaing. How old is the meteorite? \n" ); document.write( "

Algebra.Com's Answer #122164 by scott8148(6628) $\"\"$ $\"About$

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you start with 100% (1.00)\r
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\n" ); document.write( "\n" ); document.write( "after 4.2 million years, you have 50% (.50) or 1 * .5 or (.5)^1\r
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\n" ); document.write( "\n" ); document.write( "after another 4.2 million years, you have 25% (.25) or 1 * .5 * .5 or (.5)^2\r
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\n" ); document.write( "\n" ); document.write( "after N cycles (they're called half-lives) you have (.5)^N remaining\r
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\n" ); document.write( "\n" ); document.write( "(.5)^N=.076 __ taking log __ N(log(.5))=log(.076) __ dividing by log(.5) __ N=(log(.076))/(log(.5))\r
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\n" ); document.write( "\n" ); document.write( "using a calculator __ N=3.7 (approx)\r
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\n" ); document.write( "\n" ); document.write( "so the age of the meteorite is N times 4.2 million or 15.5 million (approx) \n" ); document.write( "

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