30 grams.
The definition of "half life" is the amount of time it takes for half of a sample of the substance to radioactively decay. Since the half life is 64 hours for Yttrium-90, after 64 hours, you will only have half of that isotope left, therefore there will be 120g / 2 = 60 g left after 64 hours. But you need to wait another 64 hours after that to reach 128 hours. So halve the amount again, getting 60g/2 = 30g after 128 hours.
So the original 120 gram sample will decay to 30 grams after 128 hours.
