Question 1005726
We use the formula
A(t) = Ao * (1/2)^(t/30)
.28Ao = Ao * (1/2)^(t/30)
.28 = (1/2)^(t/30)
Now take ln and get
ln .28 = (-t/30)ln 2
t = -30 * ln(.28) / ln 2 = 55.1 seconds