If we start with 400 atoms of a radioactive substance, how many would remain after one half-life ?

If it's half life, that means that half of the atoms have died out. Therefore, 200 atoms have died, so there are 200 atoms left (exactly one half of 400)

200, the half life of a radioactive substance is pretty simple to remember. It is literally cutting it in half, however, don't make the same mistake I did in this subject, the second half life does not reduce it to 0, it reduces it to 100. It just keeps half-lifing over and over again. For example: 400 atoms after one half life:200, after two: 100, after 3, 50, and so on.

A., d., and f i think