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.
The appropriate compatible numbers to use to estimate would be 20(1/2)
estimated product is 10, took the quiz :)
1) divide 26.32 by 8 = 3.29
2) 3.29 x 20= 65.80
3) he would spend 65.80 on 20 gallons of gas