Question 1104200
Time t in decades
Only needed is ratio between P and {{{P[o]}}}
e, you know


{{{e^(kt)=P/P[o]}}}

If {{{t=1}}};
{{{e^(k*1)=0.9}}}
{{{ln(e^k)=ln(0.9)}}}
{{{k*1=-0.10536}}}
{{{highlight(k=-0.10536)}}}---------and is negative, for exponential decay


Model equation becomes  {{{highlight_green(P=P[o]e^(-0.1054*t))}}}
(Depending on what accuracy you want for k)