December 13th, 2013, 07:31 AM  #1 
Newbie Joined: Dec 2013 Posts: 2 Thanks: 0  Easy Analytical Geometry
A car rental firm has the following charges for a certain type of car: $25 per day with 100 free miles included, $0.15 per mile for more than 100 miles. Suppose you want to rent a car for one day, and you know you’ll use it for more than 100 miles. What is the equation relating the cost y to the number of miles x that you drive the car? For the life of me I can't understand how to get the formula in the back of the book. I am not in school currently, but I am reviewing my Calculus in preperation for Cal II. The formula they give as an answer is: y = 0.15x + 10 I don't understand where the 10 comes from and how they incorporate the $25 charge. Please break this down for me, I'm sure I'm just over complicating it. 
December 13th, 2013, 08:52 AM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 520 Math Focus: Calculus/ODEs  Re: Easy Analytical Geometry
I would begin with: Now distribute the 0.15: Combine terms and arrange in slopeintercept form: 
December 13th, 2013, 09:24 AM  #3 
Newbie Joined: Dec 2013 Posts: 2 Thanks: 0  Re: Easy Analytical Geometry
Wow. You've definitely cleared that up for me. I should have put in a little more effort on this one. I really appreciate it, Mark. 

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