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

Answer 1

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.