Final answer:
The problem situation that does not describe a rate of change of 20 miles per gallon is situation c where Shelley travels 375 miles on 15 gallons of gasoline.
Step-by-step explanation:
To determine which of the problem situations does not describe a rate of change of 20 miles per gallon, we need to calculate the rate of change in each situation.
For situation a, Darryl uses 12 gallons of gasoline to drive 240 miles. Therefore, his rate of change is 240 miles / 12 gallons = 20 miles per gallon.
For situation b, Ramona drives 40 miles on 2 gallons of gasoline. Her rate of change is 40 miles / 2 gallons = 20 miles per gallon.
For situation c, Shelley travels 375 miles on 15 gallons of gasoline. Her rate of change is 375 miles / 15 gallons = 25 miles per gallon.
For situation d, Amber uses 1.2 gallons of gasoline to travel 24 miles. Her rate of change is 24 miles / 1.2 gallons = 20 miles per gallon.
Therefore, the problem situation that does not describe a rate of change of 20 miles per gallon is situation c, where Shelley travels 375 miles on 15 gallons of gasoline.