Write the monthly cost C (in dollars) as a function of the number x of minutes used.

Question

A cell phone plan has a basic charge of $50 a month. The plan includes 600 free minutes and charges 10 cents for each additional minute of usage.

Answer 1

C(x) = 50 + (x - 600)(0.10)

Answer 2

Write the monthly cost C (in dollars) as a function of the number x of minutes used.

A cell phone plan has a basic charge of $50 a month. The plan includes 600 free minutes and charges 10 cents for each additional minute of usage.

The 600 minutes are free and will not factor into the equation (function) that is to be written.

Using a linear equation, we can represent the $50 charge as the y-intercept of the line (read, 0 minutes of usage beyond the 600 free ones) and $.10 as the slope of the line (read, how much the monthly cost goes up for each minute beyond the 600). Then, the cost C can be represented as:

50 + .10x = C