Use the exponential decay equation, to find the half-life of a material that decays from 40mg to 30mg in 10 hours.
30 = 40e^10k
.75 = e^10k
10k = ln.75
k = ln.75/10 = -.028768
then
1/2 = e^-.028768t
- .028768t = ln .5
t = ln .5/-.028768 = 24.09
the half-life is appr. 24.1 hours