Answer
(a) $-0.087$ or $-8.7\%$.
(b) $ 45.7\ g$
(c) $ 4.1$ days.
(d) $ 8.0$ days.
Work Step by Step
Given $A(t)=A_0e^{-0.087t}=100e^{-0.087t}$, we have:
(a) The decay rate of iodine-131 is $-0.087$ or $-8.7\%$.
(b) $A(9)=100e^{-0.087(9)}\approx45.7\ g$
(c) $A(t)=100e^{-0.087(t)}=70\Longrightarrow t=\frac{ln(0.7)}{-0.087}\approx4.1$ days.
(d) $A(t)=100e^{-0.087(t)}=100/2 \Longrightarrow t=\frac{ln(1/2)}{-0.087}\approx8.0$ days.
