Answer
$89.1$ years.
Work Step by Step
1. Use the decay model $A=A_0e^{kt}$, for $t=63$, we have $\frac{A_0}{2}=A_0e^{63k}$, thus $k=\frac{ln(1/2)}{63}$
2. From 200 g to 75 g, we have $75=200e^{kt}$ and $t=\frac{ln(75/200)}{k}$, thus $t=\frac{ln(75/200)}{\frac{ln(1/2)}{63}}\approx89.1$ years.
