Uranium-235 has a half-life of 700 million years. How long does it take a 100-gram sample of Uranium-235 to decay to a mass of 50 grams?
700 million years. The half-life is the time taken for half the mass/number of atoms in a sample to decay.
700 million years
Half-Life is the time for a substance (U-235 in this case) to decay to 1/2 its original mass.
Since the problem is asking for the time for U-235 to decay to 1/2 its original mass (100 grams to 50 grams) then the decay time is 1 half-life, or 700 million years.