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March 6, 2015

Homework Help: Math - Exponential Decay

Posted by Anonymous on Saturday, January 24, 2009 at 7:40pm.

Radium decays exponentially. Its half-life is 1,690 years. How long will it take for a 50-gram sample of radium to be reduced to 5 grams?


1/2P = Pe^(-1690k)

(ln1/2)/(-1690) = k

4 x 10^-4 = k


5 = 50e^-kt

ln0.1 = -kt

ln0.1/-k = t

5,630 years = t


...but textbook answer is 5,614 years. Where did I go wrong?

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