An isotope sample initially weighing 500 grams decays to produce a daughter element. what is the amount of parent element remaining after two half-lives?

The answer is 125 grams.

Explanation:

Elements and substances decay in time and the amount of time for it to be half of it's initial amount is defined as "half-life of that substance".

In the example, the initial weight is 500 grams and we are asked to determine the amount after two half-lives, which after the first half-life is 500/2 = 250 grams and after the second one, 250/2 = 125 grams remains from the parent element.

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