Answer:
Two half lives.
Explanation:
It is known that the decay of isotopes and radioactive material obeys first order kinetics.
Also, it is clear that in first order decay the half-life time is independent of the initial concentration.
That means for a sample 100% to decay to 50 % it will take one half-life, and to decay the remaining 50% to 25% it will take another half-life.
So, for a sample has 25% parent and 75% daughter it will have two half-lives.
