Answer
There will be around 0.8 gram out of the 100-gram sample after 500 years.
Work Step by Step
This is about half-life so it involves exponential decay.
RECALL:
Exponential decay is represented by the formula
$y=C(1-r)^x$
where
C = initial/original amount
r = decay rate
x = number of time intervals
The given situation has:
C = 100 grams
r = $50\%$ per 72 years
x = $\frac{500}{72} = \frac{125}{18}$
Substitute these values into the given formula above to have:
$y=100(1-50\%)^{\frac{125}{18}}
\\y=100(1-0.5)^{\frac{125}{18}}
\\y=100(0.5^{\frac{125}{18}})
\\y \approx 0.8$
