Please refer to the universal key trading formula below
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(SkinMarketPrice * 0.8) / KeyPrice = Keys You get
For example   AK-47 Cartel Minimal Wear is 3.72 USD and the Universal Price of keys are 2.5usd, Therefore the equation is
(3.67$ * 0.8) / 2.5$ = 1 Key  therefore  the Cartel Skin in Keys is equivalent to 1 Key.
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If you are too lazy to calculate by yourself dont worry , I can calculate for you or you could use the available rates below
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AK-47 | Cartel (Minimal Wear) = 1 Key
AWP | Phobos (Factory New) = 1 Key
M4A4 | 龍王 (Dragon King) (Minimal Wear) = 2 Keys
AK-47 | Blue Laminate (Minimal Wear) =  1 Key
AWP | Sun in Leo (Well-Worn) = 1 Key