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

The amount of a decaying substance in any time t (at) is calculated by the equation, 
                                           at = (a1) x (1/2)^h

where a1 is the original amount and h is the number of half-lives.

Substituting the given values with a1 as the unknown,
                              20 g = (a1) x (1/2)^3    ; a1 = 160 g

Thuss, there were 160 grams of the isotope at the beginning. 
 

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