Asample of bismuth contains 210 decayed 33% of original mass after 8 days find half life of the element find the mass remaining after 12 days

A sample of bismuth-210 decayed to 33% of its original mass after 8 days.
(a)Find the half-life of this element.
(b)Find the mass remaining after 12 days 
Equation:
m(t)= I*e^-rt 
I=Initial mass
r=rate of decay
t= time 
Since it is half-life, 1/2= e^-rt
rate= (ln 2)/ h
h= number of days to decay half of the mass
When I had this question I wasn't given h and r because working with two variables is confusing. 
So t=8/365 and since t represents years and 8 days out of 365 days it gives you 8/365. I hope this helps. Good luck!

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