Answer:
814.5 mg remain after 4 hours
Explanation:
The multiplier of the original quantity is 1 -5% = 0.95 for each passing hour. The amount remaining after x hours is ...
y = 1000·0.95^x
For x=4, the amount remaining is ...
y = 1000·0.95^4 = 1000·0.81450625
y ≈ 814.5 . . . mg remain after 4 hours