This is about half life of elements with exponential decay.
Half life = 20 years
We are given;
y = ab^(t)
Where;
t is the half life
y = a/2 is the amount of substance remaining after decay
a is amount of substance initially
b = 100% - 3.411% = 96.589% = 0.96589
a/2 = a(0.96589)^(t)
a will cancel out to give;
0.5 = 0.96589^(t)
ln (0.5) = t(ln 0.96589)
t = ln(0.5)/ln(0.96589)
t = 19.968 days
This is approximately 20 days.
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