In 1.2 years half of 64 mg of radioactive isotopes decays. The amount present after 6 years is?

After 1.2 years, half of the 64 mg of radioactive isotopes decays, leaving 32 mg. After 6 years, another half-life period of 1.2 years occurs five times, with each decay halving the remaining amount. Therefore, after 6 years, the amount present will be (64 \text{ mg} \times \left(\frac{1}{2}\right)^5 = 1 \text{ mg}).