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# How to solve a half-life problem?

I'm not so sure how to solve these: If you have a fossil that contains 3,125 parent isotopes and 96,875 daughter isotopes, how many half lives have passed? And If you have a fossil that contains 3,125 parent isotopes and 96,875 daughter isotopes and it takes .5 million years for a half life, how old is the rock?

### 2 Answers

- obscuritas88Lv 47 years agoFavorite Answer
3125/96875 = Percent remaining

=0.0323=3.23%

(0.5)^n = 0.0323

n*log(0.5)=log(0.0323)

n = log(0.0323)/log(0.5) = 4.95 half lives have passed

If 0.5 million years = 1 half life

4.95 * 0.5 = 2.475 Million years old

- 4 years ago
to remedy one million/2 existence issues, you need to be certain what proportion circumstances the substance is halved. Cs-137 has a one million/2 existence of 30 years. you have 1mg of it. You wait ninety years. After ninety years, it would decompose thrice, subsequently, one million/ (2)^3= one million/8 mg Cs137