Tritium has a half life of 12.5 years against beta decay. What fraction of a sample of pure tritium will remain undecayed after 25 years?

Explanation

By definition of half-life, half of the initial sample will remain
undecayed after 12.5 y. In the next 12.5 y, one-half of these nuclei
would have decayed. Hence, one fourth of the sample of the initial
pure tritium will remain undecayed.