Answer:
5.35 grams
Explanation:
Given that a 12 grams of a substance will decay exponentially to 7 grams in 2 hours, you want to know the amount remaining after 3 hours.
Equation
The amount remaining can be described by ...
remaining = (initial amount)·(decay factor)^(t/(decay interval))
remaining = 12(7/12)^(t/2) . . . . where t is in hours
Application
After 3 hours, the amount remaining is ...
remaining = 12(7/12)^(3/2) ≈ 5.35 . . . . grams
About 5.35 grams will remain after 3 hours.
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Additional comment
The half-life is about 2.572 hours.