Question

Haley pays a monthly fee of $20 for her cell phone and then pays 5 cents per minute used. The total cost of Haley’s monthly cell phone bill can be expressed by the function C(m) = 0.05m + 20, where m is the number of minutes used. What are the domain and range of the function C(m)?

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Answer 1

Domain: m ≥ 0 (m ∈ [0, ∞)) — you cannot use a negative number of minutes.
Range: C(m) ≥ 20 (C ∈ [20, ∞)) — the minimum bill is $20 (when m = 0) and increases without bound as m increases.

(If you require whole minutes, the domain is the nonnegative integers and the range is {20 + 0.05m : m is a nonnegative integer}.)