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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?

  • Algebra -

    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.

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