Answer
Time Intervals: 3.333
Final Amount after $x$ time intervals: 19.9
Is your Final Amount Reasonable?: yes
Work Step by Step
time intervals
40 years
12 years to a half-life
$40/12$
$10/3$
$3.33$
final amount
$200*.5^{3.33}$
$200*.09944$
$19.888$
$19.9$
