not sure how to start this oneIn 660 days unknown radioactive substance decay to 33 percent of its size.
1)What is the half life of this substance?
t=
2)How long will it take for a sample of 100mg to decay to 60 mg?
T=
Assume that you start with 1 unit, then after 660 days, you will have 0.33 units left
so use $\displaystyle 0.33 = e^{-660r}$ to find $\displaystyle r$
then use the formula $\displaystyle t_h = \frac { \ln 2}{r}$, where $\displaystyle t_h$ is the half-life
i think you will be able to handle the next question after getting this