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K is the equilibrium constant so the numbers you use are the equilibrium concentrations of the products and reactants. This tells you that this ratio will be approached when a reaction reaches equilibrium from its initial concentrations.
Adding on, K is referring to the concentrations held to the specific power and ratio as stated in the balanced equilibrium equation given in the problem. Knowing the concentrations won't do you any good if they aren't held to the correct power within the fraction for K.
It also matters if it's Kc or Kp. While Kc does refer to the ratio of concentrations to their respective powers of products and reactants, Kp will refer to the ratio of partial pressures to their respective powers of products and reactants.