A radioactive sample contains 10,000 atoms. after two half-lives, how many atoms-of any type-remain in the sample?

A radioactive molecule will gradually decay so it would lose some it mass. The speed of the decay is different for every molecule and it was stated as half-lives. For every half-lives, a radioactive molecule will decay 50% or half of its weight.

Then, the number of molecules remains from 10,000 molecule that undergo 2 half-lives: 10,000 * (1/2) ^2 = 10,000 * 1/4 = 2,500 molecules