if a substance has a half-life of 10 million years, what fraction of the atoms
remain after 20 million years?

Answer is 1/4 of the atoms

at 0 years it would be the full amount (X)

at 10 million years you have half: X/2

at 20 million years you have 1/4 because: (X/2)/2 will equal 1/4

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.

D.attorney because an attorney can in this situation