Problem Elimination of a drug from the metabolism

The elimination of a drug like Acetylsalicylic acid (aka aspirin) from the metabolism might follow a zero order reaction. You take 200 mg of the drug and every 10 minutes a blood sample is tested for the remaining amount of the drug. 

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We are looking at the decrease in the amount of the drug like Aspirin and therefore can write A(spirin) —  

Of course we could draw a graph from our blood sample data and extrapolate the line to 125 minutes. 

Instead, let s write the rate equation for this reaction, bearing in mind that because we are looking at a decrease in A the rate must be negative  

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