Final answer:
After 25 years, 18.75g of a 100g sample of Co-60 will have decayed.
Step-by-step explanation:
The half-life of Co-60 is 5 years, which means that after 5 years, only 50% of the original sample will remain. Using this information, we can calculate the amount of Co-60 that will have decayed after 25 years.
Since the half-life is 5 years and 25 years is 5 times the half-life, we can divide 100g by 2^5 (since each half-life reduces the amount by half) to find the amount that will remain.
Therefore, after 25 years, 18.75g of the 100g sample of Co-60 will have decayed.