Question

Carbon–14 is a radioactive isotope that decays exponentially at a rate of 0.0124 percent a year. How many years will it take for carbon–14 to decay to 10 percent of its original amount? The equation for exponential decay is At = A0e–rt.

Answer 1

100-0.0124=99.9876

10%=1/10
1/10P=P(0.999876)^t
divide both sides by P
1/10=0.999876^t
take ln of both sides
ln(1/10)=t (ln(0.999876))
divide both sides by (ln(0.999876))
ln(1/10)/(ln(0.999876))=t
use calculator
18568.08=t
about 18568 years