A:   The reaction quotient, Q has the same form as the equilibrium constant $K_{eq}$ and is evaluated using any given concentration of the species involved in the reaction, and not necessarily equilibrium concentrations.
R:    If the numerical value of Q is not the same as the value of equilibrium constant, then the system is at equilibrium

$\mathrm{Q}=\frac{\left[\mathrm{D}\right]\left[\mathrm{C}\right]}{\left[\mathrm{A}\right]\left[\mathrm{B}\right]}$ $\left[\mathrm{D}\right],\left[C\right]$ are the concentrations at any given time.

$\mathrm{K}=\frac{\left[\mathrm{D}\right]\left[\mathrm{C}\right]}{\left[\mathrm{A}\right]\left[\mathrm{B}\right]}$$\left[\mathrm{D}\right],\left[C\right]$ are the concentrations at equilibrium.

Q can be used to determine which direction a reaction will shift to reach equilibrium. If K > Q, a reaction will proceed forward, converting reactants into products. If K < Q, the reaction will proceed in the reverse direction, converting products into reactants. If Q = K then the system is already at equilibrium.