Answer:
That is probably tellurium 134 which has a half-life of 41.8 minutes but we'll stick with the 40 minutes you stated.
240 / 40 minutes equals precisely 6 half-lives.
Every half-life reduces a radioactive substance by one half.
So 6 half-lives reduces it by 1 / (2^6) which equals 0.015625
576 * .015625 = 9 grams exactly.
Step-by-step explanation: