Question 325438
{{{N=N0*e^(kt)}}}
Find k.
{{{19.9=20*e^(k(25))}}}
{{{e^(25k)=19.9/20}}}
{{{25k=ln(19.9/20)}}}
{{{k=ln(19.9/20)/25}}}
.
.
.
Now find the half life. 
{{{N0/2=N0*e^(kt)}}}
{{{kt=ln(1/2)}}}
{{{t=ln(1/2)/k}}}
{{{t=(25*ln(1/2))/(2*ln(19.9/20))}}}
{{{t=1728.5}}}
The half life is 1728.5 years.