The half-life of a radioactive substance is given 3 hours.
Given the initial amount of substance is 800 grams. After 3 hours, the substance becomes half that is 400 grams. Then again after 3 more hours, the substance becomes half again that is 200 grams. Again after three hours, the substance becomes half that is 100 grams.
Thus, the amount of radioactive material after 9 hours is 100 grams.