Calc
posted by Lee .
A sample of a radioactive substance decayed to 93.5% of its original amount after a year.
a) What is the halflife of the substance?
? years
(b) How long would it take the sample to decay to 10% of its original amount?
? years

a) Let the half life in years be H
2^(1/H) = 0.935
Solve that for H.
1/H log 2 = log 0.935
1/H = 0.09696
H = 10.31 years
b) Solve 2^(t/10.31) = 0.10
t will be the time required to decasy to 10% activity
You could also solve
(0.935)^t = 0.1
The answer should be the same either way you do it: about 30 years. 
Thank you!