Question 163533
The isotope follows this power law as a function of time,
{{{N(t)=N[0](.9975)^t}}}
where t is measured in years and {{{N[0]}}} is the initial amount of the isotope.
Half life is when {{{N(t)=(1/2)N[0]}}},
{{{(1/2)N[0]=N[0](0.9975)^t}}}
{{{(0.5)=(0.9975)^t}}}
{{{t=log(0.9975,0.5)}}}
{{{t=276.9}}}
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Looks like b) 277 years is the closest answer.