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# Reverse Plugging In: Hard to Spot RPI Situations

In a phase III clinical trial of Dosaxin, a new drug, patients received a progressively growing dosage of Dosaxin for a few days. On the first day, each patient received 15 milligrams of Dosaxin. On each of the following days, the daily dosage was $$m$$ milligrams greater than the dosage received the day before, reaching a dosage of 43 milligram on the last day of the trial. For how many days did the trial last if each patient received a total amount of 145 milligrams of Dosaxin during the whole trial?
Incorrect. [[Snippet]]
Incorrect. [[Snippet]]
Incorrect. [[Snippet]]
Incorrect. [[Snippet]]
Correct. [[Snippet]] In this case, the average is >$$\displaystyle \text{Average} = \frac{43 + 15}{2} = \frac{58}{2} = 29$$ Use the average and the sum to calculate the number of days: >$$\text{Sum} =\text{Average} \times \text{Number of numbers}$$ >$$145 = 29 \times \text{Number of days}$$ Thus, the number of days is >$$\displaystyle \text{Number of days} = \frac{145}{ 29} = 5$$
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