What percentage of a radioactive substance remains after two half-lives?

Answer: 25%.

Explanation:

The decay of a radioactive substance obeys the rules of first order reaction. In first order reaction half life time (t1/2) is constant and does not depend on the initial concentration of the substance (t1/2 = 0.693/k) where k is the rate constant. Half-life time (t1/2) is the time needed for the reaction that the initial concentration of the reactant (substance) reaches its half (reaction be completed by 50 %). radioactive substance (100%) (first half life) → 50 % (second half life) → 25 % Hence, the radioactive substance will reach 25 % of its concentration after two half-lives.

25%

I know this because i have a doctorates in Percents