It would take 500 minutes for the cost to be $21.50.
A linear equation is in the form:
y = mx + b
where y, x are variables, m is the rate of change (slope) and b is the y intercept.
Let y represent the cell phone bill for x minutes. Using points (230, 17.45) and (370, 19.55):
[tex]y-y_1=\frac{y_2-y_1}{x_2-x_1} (x-x_1)\\\\y-17.45=\frac{19.55-17.45}{370-230} (x - 230)\\\\y=0.015x+14[/tex]
For 150 minutes:
y = 0.015(150) + 14 = 16.25
For $21.50:
21.50 = 0.015x + 14
x = 500
It would take 500 minutes for the cost to be $21.50.
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