In this situation, we can define the variables in the equation \( y = 4 \cdot x \) as follows:
- \( x \): This variable represents the time in minutes that have passed while Sarah is filling the aquarium.
- \( y \): This variable represents the total number of gallons of water that have been added to the aquarium after \( x \) minutes.
So, the equation \( y = 4 \cdot x \) states that the total gallons of water \( y \) added to the aquarium is equal to the rate at which Sarah fills the aquarium (4 gallons per minute) multiplied by the time in minutes \( x \).