Question

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

Answer 1

Half life is the time taken to half the initial amount of sample.

Let the original to be x gram.
After one half life, you half of the x gram, i.e. you get 0.5x gram.
Then another half life again and you will get half of the 0.5x gram, i.e. you get 0.25x gram.
0.25x=10
x=40g