A radioactive substance's half-life is the amount of time needed for half (or 50%) of the substance to decay. Let's say we have a radioactive substance with a half life of 20 years. (a) what percent of the substance would be radioactive after 40 years?

25 percent of the substance would be radioactive after 40 years.

Step-by-step explanation:

Initial mass of the substance =

Half life of the substance =

Mass of substance left after 40 years = N

t = 40 years

Radio active decay follows first order kinetics, so:

Percent of the substance would be radioactive after 40 years:

the solution is h=10 because you add 9 to each side of the equation.