Final answer:
The half-life of strontium-90 is 28 years. To ensure that only 1/16 of the original sample remains as radioactive, the sample must be stored for 112 years.
Step-by-step explanation:
The half-life of strontium-90 is 28 years. To calculate the time required for only 1/16 of the original sample to remain as radioactive, we need to determine the number of half-lives that have passed. Since each half-life is 28 years, we can divide the total time by the half-life to find the number of half-lives: 1/16 = 2^(-n), where n is the number of half-lives. Solving for n, we find that n = 4. Therefore, the sample must be stored for 4 x 28 years, or 112 years, to ensure that only 1/16 of the original sample remains as radioactive.