In 10 years, 25% of a radioactive substance decays. What is its half-life?

A. 25 years
B. 24 years
C. 20 years
D. 29 years

To determine the half-life of the radioactive substance, we need to find the amount of time it takes for 50% of the substance to decay. Given that in 10 years, 25% of the substance decays, it means that after 10 years, we are left with 75% of the initial amount.

Next, we need to find out after how many years the substance decays to half of its remaining amount, i.e., when it decays to 37.5% of the initial amount (50% of 75%). Since the substance decays by 25% in 10 years, we can divide 10 years by 25% to find the time it takes for a 25% decay.

10 years ÷ 25% = 40 years.

Therefore, it takes 40 years for the substance to decay to half of its remaining amount. This is the half-life of the substance.

Among the options provided, none of them correspond to our calculated half-life.