Question 1005553
We use
A(t) = Ao*(1/2)^(t/5600)
.12Ao = Ao*(1/2)^(t/5600)
.12 = (1/2)^(t/5600)
ln .12 = (t/5600) * ln (1/2)
t/5600 = ln .12 / ln (1/2)
t = 5600(ln .12 / ln (1/2))
t = 17130 years