After four half-lives 0.1 mole of the radioactive isotope would remain. If the amount of the isotope that remains after each half life is one half of previously existing amount. Half of 1.6 equals 0.8, half of 0.8 equals 0.4, half of 0.4 equals 0.2, and finally half of 0.2 equals 0.1 mole.
