To determine the number of photos that can be printed for 1 dollar, we would determine the slope.
Let x represent number of photos printed
Let y represent total cost of photos in dollars
Slope = (y2 - y1)/(x2 - x1)
From the table,
x1 = 10, x2 = 20
y1 = 2, y2 = 4
Slope = (20 - 10)/(4 - 2)
Slope = 10/2 = 5
5 photos can be printed for 1 dollar