Question 1006959
We use the formula
A(t) = Ao*(1/2)^(t/5730)
.6Ao = Ao*(1/2)^(t/5730)
.6 = (1/2)^(t/5730)
ln .6 = ln (1/2)^(t/5730)
ln .6 = (t/5730) * ln (1/2)
t/5730 = (ln .6)/(ln 1/2)
t = 5730*(ln .6)/(ln 1/2) = 4223 years