Answer
(a) $-0.0244$ or $-2.44\%$.
(b) $ 391.7\ g$
(c) $ 9.1$ years.
(d) $ 28.4$ years.
Work Step by Step
Given $A(t)=A_0e^{-0.0244t}=500e^{-0.0244t}$, we have:
(a) The decay rate of strontium-90 is $-0.0244$ or $-2.44\%$.
(b) $A(10)=500e^{-0.0244(10)}\approx391.7\ g$
(c) $A(t)=500e^{-0.0244t}=400\Longrightarrow t=\frac{ln(4/5)}{-0.0244}\approx9.1$ years.
(d) $A(t)=500e^{-0.0244t}=500/2 \Longrightarrow t=\frac{ln(1/2)}{-0.0244}\approx28.4$ years.
