# Math Help - Half-life

1. ## Half-life

In 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=
not sure how to start this one

2. Originally Posted by viet
In 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=

not sure how to start this one
Assume that you start with 1 unit, then after 660 days, you will have 0.33 units left

so use $0.33 = e^{-660r}$ to find $r$

then use the formula $t_h = \frac { \ln 2}{r}$, where $t_h$ is the half-life

i think you will be able to handle the next question after getting this