Can you help me understand the equation of how to find the CM per…

Can you help me understand the equation of how to find the CM per unit of scarce resource
Hat Scarf Mittens
Selling price per unit 15.00 15.00 15.00
Cost of yarn per unit $ 4.00 $ 6.00 $ 4.00
Contribution Margin 11 9 11
Time in minutes 20 30 40

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