HalfLife
posted by Noah .
Uranium238 decays to Lead206. The half life of Uranium238 is 4.5 billion years. If you have a 100 g sample. How much sample will you have after 2 half lifes?

just do it in your head ...
after 1 halflife you would have 50 g left
after 2 halflifes you would have 25 g left 
Your question isn't quite clear to me. If you want the mass of U after two half lives, then 25 g is correct. If you want the mass of sample left after two half lives, that is 25g U + the equivalent of 75 g U that's been converted to Pb. That conversion is 75 g U x (atomic mass Pb/atomic mass U) = about 75*207/238 = about 65 g. Therefore, the total mass is 25 + about 65 = about 90 g.

If the halflife of a radionuclide is 1 month, is a sample of it completely decayed after 2 months?