From the given graph, let's find the slope.
Take two points on the line:
(x1, y1) ==> (10, 4)
(x2, y2) ==> (20, 8)
Apply the sloe formula to find the slope:

Thus, we have:

The slope of the line is 2/5.
Since the x-axis of the graph represents the amount of Yellow food colouring in ounce while the y-axis represents the red food colouring, the slope represents the amount of red food colouring needed per ounce of yellow colouring.
For every ounce of yellow food colouring, there is 2/5 ounce of red food colouring.
ANSWER:
Slope = 2/5
For every ounce of yellow food colouring, there is 2/5 ounce of red food colouring.