The half-life of carbon-14 is 5,730 years. how many grams of parent material will remain from a 120-gram sample of carbon-14 after 2 half-lives?
90
60
30
15

After one half-life . . . 1/2 of the total parent material

After another half-life . . . 1/2 of the half remains = 1/4 of the total original parent

If the original parent is 120 grams, then (1/4 x 20) = 30 grams remains
after 2 half-lives.  It makes no difference what the substance is, or
how long its half-life is.