Question 1005725
The half-life IS three days, since half of it is gone in that time.
From there we use
A(t) = Ao * (1/2)^(t/3)
.3Ao = Ao * (1/2)^(t/3)
.3 = (1/2)^(t/3)
.3 = 2^(-t/3)
ln .3 = (-t/3) ln 2
ln .3 / ln 2 = -t/3
t = -3ln .3 / ln 2 = 5.21 days