Answer:
a. 
b. 
Step-by-step explanation:
First, we need tot find a general expression for the amount of caffeine remaining in the body after certain time. As the problem states that every hour x percent of caffeine leaves the body, we must substract that percentage from the initial quantity of caffeine, by each hour passing. That expression would be:

Then, to find the amount of caffeine metabolized per hour, we need to differentiate the previous equation. Following the differentiation rules we get:

The rate is negative as it represents the amount of caffeine leaving the body at certain time.
Factoring out the 1/2 (that is the coefficient of the variable) would require you to divide the equation by one half, which is the same as multiplying by two. After doing this, the answer can be expressed as:
1/2 (d+12)
8*4=32 :) I hope this helps!
The answer to the question is <span>70.6179775281. which also can be converted to 71.</span>