This mineral crystallize after 83 years
For each half-life, you divide the total amount of the isotope by 2, so
Amount remaining = original amount/2^n
n the number of half-lives.
A = Ao/2^n
If there were originally 100 atoms, and 75 of the atoms remain, we have
100/75 = 2^n
4/3 = 2^n
log(4/3) = n log 2
n = log(4/3)/log2 = 0.1249/0.3010 = 0.415
t = 0.415*t/2 = 0.415*200 = 83 years
This mineral crystallize after 83 years
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