Aradioactive 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

The correct name for n3-ion should be azide.