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You anons are such a bunch of dumbfucks.
Think about it.
You layout 9 for the car. This is your initial purchase price. This is where you begin. You finally sell that car for 13. This is your exit point. So, on your journey (it helps to think of economics like this) you changed value from 9 to 13, or if you prefer in simple terms, you 'earned' 4k.