Question 941762
What is the rule of 70?
Use the half-life to find the decay constant in {{{y=Ie^(-kt)}}}.
You know that {{{(1/2)=1*e^(-k*140)}}},
{{{1/2=e^(-k*140)}}}
{{{ln(1/2)=-k*140 }}}
{{{ln(2)=k*140}}}
{{{k=(1/140)ln(2)}}}


{{{k=0.00495}}}


{{{y=Ie^(-0.00495t)}}}-----reminder, this is exponential.  No average makes sense here, unless for some limited time period.