Victoria's method is linear because the number of minutes increase by an equal number (15) every month.
Workings in the attachments below. The green line is the function that has been set up for Victoria. The lines that form a curved looking graph belong to the function that was set up for Zach.
Workings in the attachments below. The green line is the function that has been set up for Victoria. The lines that form a curved looking graph belong to the function that was set up for Zach.