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.
