The half-life of a radioactive kind of tellurium is 40 minutes. How much will be left after 240 minutes if we started with 576 grams of it?

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.