Answer
Time Intervals: 3.3
Final Amount after $x$ time intervals: 2.1
Is your Final Amount Reasonable?: yes
Work Step by Step
time intervals
500 years
152 years to a half life
$500/152$
$3.289$
$3.29$
$3.3$
final amount
$21*.5^{3.3}$
$21*.1015$
$2.132$
$2.1$
