Posted by Mario on .
The level of thorium in a sample decreases by a factor of onehalf every 2 million years. A meteorite is discovered to have only 8.6% of its original thorium remaining. How old is the meteorite?

Algebra 
Dr Russ,
After 1 half life it will have 0.5 (or 1/2^1) remaining
After 2 half lifes it will have 0.25 (or 1/2^2)remaining
After 3 half lifes it will have 0.125 (or 1/2^3) remaining
After n half lifes it will have 1/2^n remaining
There is 0.086 remaining so
0.086 = 1/2^n
or 2^n=1/0.086
solve for n to give you the number of half lives
and n x 2 x 10^6 y = age of meteorite.