Question

Earth Science - After three half-lives, a sample contains 20 grams of parent isotopes. How many grams of the parent isotope were present to start?

ANSWERS:

320

160

80

40

Answer 1

Let there be 'x' grams of isotopes at the beggining.
No. of grams remaining after first half life=1/2*x
No. of grams remaining after second half life=1/4*x
No. of grams remaining after third half life=1/8*x
Now,1/8*x=20(given)
=>x=160grams