Substance X has a radioactive half-life of 12 years. How much time must have elapsed if only 9 grams is left from an original sample of 150 grams?
Each arrow indicates one half-life, for a total of four half-lives. Since one half-life is 12 years, the total time elapsed is 12 x 4 = 48 years. The exact answer is a little above 48 years. But the approximation is correct since the choices are far apart and no values higher than 48 are given.