document.write( "Question 600575: The half-life of 234U, uranium-234, is 2.52 105 yr. If 98.9% of the uranium in the original sample is present, what length of time (to the nearest thousand years) has elapsed?
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Algebra.Com's Answer #379425 by ankor@dixie-net.com(22740)\"\" \"About 
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The half-life of 234U, uranium-234, is 2.52 105 yr.
\n" ); document.write( "Probably mean 2.52(10^5) yrs.
\n" ); document.write( " If 98.9% of the uranium in the original sample is present, what length of time (to the nearest thousand years) has elapsed?
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\n" ); document.write( "The radioactive decay formula:
\n" ); document.write( "A = Ao*2^(-t/h),
\n" ); document.write( "where:
\n" ); document.write( "A = resulting amt after t time
\n" ); document.write( "A0 - initial amt (t=0)
\n" ); document.write( "t = time of decay
\n" ); document.write( "h = half-life of substance
\n" ); document.write( ":
\n" ); document.write( "Let initial amt = 1
\n" ); document.write( "1*2^(-t/(2.52(10^5)) = .989
\n" ); document.write( "use common logs
\n" ); document.write( "\"-t%2F%282.52%2810%5E5%29%29\"*log(2) = log(.989)
\n" ); document.write( "\"-t%2F%282.52%2810%5E5%29%29\" = \"log%28.989%29%2Flog%282%29\"
\n" ); document.write( "Find the logs
\n" ); document.write( "\"-t%2F%282.52%2810%5E5%29%29\" = -.01596
\n" ); document.write( "t = -.01596*-2.52(10^5)
\n" ); document.write( "t = 4021.3 ~ 4000 yrs
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