Media Services charges 
20/month for its economy plan. Find a linear model that determines the total cost, C(t), of operating a Media Services phone for t months.
Use the model to find the total cost for 4 months of service.
Solution:-
Formulate a linear model that can be used to determine the total cost, C(t), of operating a Media Services phone for t months.
The total cost involves the initial purchase of a phone plus a monthly fee of %20. The initial purchase cost of %40 is a one-time charge which is a constant term in the model.
The monthly fee is a variable cost based on the number of months of months of service. Remember that t represents the number of months of service for a phone.
The model equation would be C(t) = 20t + 40.
To find the total cost for 4 months of service, substitute 4 for t and evaluate c(4).
C(4) = 20(4) + 40
C(4) = 120
Therefore, the cost of operating a phone for 4 months is $120.