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Answer:
4 hours
Step-by-step explanation:
There are several ways to work this. One way that often works well with multiple choice questions is "try the answers".
<u>6.5 hours</u>
If the half-life is 6.5 hours, then 200 mg will remain after 6.5 hours, and 100 mg will remain after 13 hours. The decay rate is faster than that, so 6.5 hours is not right, but a lesser value is.
<u>4 hours</u>
If the half-life is 4 hours, then 200 mg will remain after 4 hours, and 100 mg will remain after 4 more (=8) hours. An additional 4 hours will cut that in half again to 50 mg after 12 hours.
The half-life is 4 hours.
