Question 324617
{{{N=N0e^(kt)}}}
{{{ 4=5e^(k(30))}}}
{{{e^(30k)=4/5}}}
{{{30k=ln(4/5)}}}
{{{k=ln(4/5)/30}}}
{{{k=-0.007438}}}
.
.
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{{{e^(kt)=1/2}}}
{{{kt=ln(1/2)}}}
{{{t=ln(1/2)/(-0.007438)}}}
{{{t=93.2}}}
The half life is 93.2 seconds.