In a closed system at equilibrium, the following reaction takes place: A + B ⇌ C + D. The concentrations of the reactants and products at equilibrium are as follows: [A] = 0.2 M, [B] = 0.2 M, [C] = 0.5 M, [D] = 0.5 M. What is the equilibrium constant (Keq) for this reaction at the given concentrations?