The Number of gallons of gasoline sold at price dollars per gallons is .

Rate of change in the number of gallons of gasoline is .

(a)

Meaning of :

represents rate of change in the number of gallons at dollars per gallons.

(b)

:

is negative as the price per gallons increases, the number of number of gallons if gasoline decreases.

(a) represents rate of change in the number of gallons at dollars per gallons.

(b) is negative.