Question 928743
Decay equation {{{p=18*e^(-kt)}}} and you first want to take the given data and use to find k.  t is time in years, p is quantity remaining after time t.


{{{ln(p)=ln(18)-kt}}}
{{{ln(p)-ln(18)=-kt}}}
{{{-k=(ln(p)-ln(18))/t}}}
{{{k=(ln(18)-ln(p))/t}}}
Substitute the given data point:
{{{highlight_green(k=(1/5)ln(2))}}}, and you can compute this value yourself of k.



How much is p at t=11?
You know k now, and given t.
Evaluate p.