If We Start With 400 Atoms Of A Radioactive Substance, How Many Would Remain After Two Half-Lives?

Two half-lives produce (1/2) x (1/2) = 1/4 of the decaying substance.

There would still be 400 atoms. But 300 would have thrown off

particles from their nucleuses, and only 100 would still be radioactive.

You basically take a half-life one at a time. The first half-live gets rid of 200 of the atoms, leaving you with 200. Then the second half life gets rid of 100 of the remaining atoms (half of the 200) leaving you with 100 atoms