A maker of light &cheap trucks offers three different build materials for the same model of…

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A maker of light &cheap trucks offers three different build materials for the
same model of vehicle. The exchange constant for weight saving is $12/kg. The
first material is steel, the second is Aluminum, which costs $2500 more, but
weighs 300kg less, and the third is carbon-fiber, which costs $8000 more, but
weights 500kg less. Which is the best buy?