Answer:
y = 100x - 10
Explanation:
Let 'x' be the number of tennis balls, therefore:
50x = $30 -> x = $0.6
40x = $20 -> x = $0.5
Therefore, we have the following two points:
(0.6, 50) and (0.5, 40)
The slope is:
m = (y2 - y1)/(x2-x1) = 10/0.1 = 100
The equation of the line is:
y - y0 = m(x-x0)
y = 100(x-0.5) + 40
y = 100x - 10