document.write( "Question 808944: The half-life of radium is approximately 1690 years. If a laboratory has 50 mg of radium, how long will it take for the substance to decay to 40 mg, to the nearest 10 years? \n" ); document.write( "
Algebra.Com's Answer #487514 by ankor@dixie-net.com(22740)\"\" \"About 
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The half-life of radium is approximately 1690 years.
\n" ); document.write( " If a laboratory has 50 mg of radium, how long will it take for the substance to decay to 40 mg, to the nearest 10 years?
\n" ); document.write( ":
\n" ); document.write( "The radioactive decay formula: A = Ao*2^(-t/h), where
\n" ); document.write( "A = Amt remaining after t time
\n" ); document.write( "Ao = 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( "50*2^(-t/1690) = 40
\n" ); document.write( "Divide both sides by 50
\n" ); document.write( "2^(-t/1690) = \"40%2F50\"
\n" ); document.write( "2^(-t/1690) = .8
\n" ); document.write( "using natural logs
\n" ); document.write( "ln[2^(-t/1690)] = ln(.8)
\n" ); document.write( "\"-t%2F1690\"ln(2) = ln(.8)
\n" ); document.write( "\"-t%2F1690\" = \"ln%28.8%29%2Fln%282%29\"
\n" ); document.write( "using the calc
\n" ); document.write( "\"-t%2F1690\" = -.3219
\n" ); document.write( "t = -1690 * -.3219
\n" ); document.write( "t = 544 ~ 540 yrs
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