Final answer:
Starting with 1200 atoms of a radioactive substance, after three half-lives, 150 atoms would remain as each half-life reduces the number to half of its previous amount.
Step-by-step explanation:
If we start with 1200 atoms of a radioactive substance, after three half-lives, the number of atoms remaining would be 1200 ÷ (23 = 8), which gives us 150 atoms. Each half-life reduces the number of radioactive atoms to half of the previous amount. After one half-life, we would have 600 atoms remaining; after two half-lives, 300 atoms; and after three half-lives, 150 atoms would remain of the original 1200. This progressive reduction illustrates the exponential decay characteristic of radioactive substances.