The cost to rent skis at a local sporting goods store is $15 plus $20 per day. which equation models the relationship between the total cost to rent, c, and the length of the rental in days, d?

The total cost for renting skis is the initial payment of $15 and additional payment each day. if we let d be the number of days the skis are rented then the total additional cost is 20d. the equation relating the total cost (c) is,    
                                           
                                    c = 15 + 20d

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