The half-life of tritium, or hydrogen-3, is 12.32 years. after about 24.6 years, how much of a sample of tritium will remain unchanged?

25% because the half life is 12.3 years roughly, and after 2 "cycles" (12.3 times 2) the tritium would have been reduced by 50% during the first cycle, and 50% again during the second, which would give you 25% left.