Answer :
Final answer:
It takes 120 minutes for the count rate of a radioactive element with a half-life of 40 minutes to drop from 1000 per minute to 125 per minute, as it will halve three times.
Explanation:
To calculate the time it will take for the count rate to drop to 125 per minute from an initial count rate of 1000 per minute for a radioactive element with a half-life of 40 minutes, we use the concept of half-lives. After one half-life (40 minutes), the count rate would be halved from 1000 to 500 per minute; after two half-lives (80 minutes), it would halve again to 250 per minute; and after three half-lives (120 minutes), it would halve once more to 125 per minute.
Therefore, the correct answer is B: 120 minutes.
