Question 258093
Use the given information to find the rate of decay and then you can use that rate in your half life formula.
so, you will get {{{.33=e^(rt)}}}, for the information that it gives you (decayed 33% of its original mass after 8 days).
Next we can use the 8 days information to fill in t and then solve for r.  so t=8/365 (since t represents years and 8 days out of 365 days gives you 8/365)
{{{.33=e^(r(8/365))}}}
ln of both sides gives you 
{{{ln .33=r(8/365)}}}
{{{(365* ln (.33))/8=r}}}
Now plug this rate into your formula that you are setting up for the half life.  I'll let you finish it.