At equilibrium the forward and reverse reactions run at the same rate, so the ratio of product to reactant settles at the equilibrium constant K = [B] / [A]. For a fixed total amount C = [A] + [B], the equilibrium concentrations are [A] = C / (1 + K) and [B] = C * K / (1 + K). Raise K to favor products, or change the total amount, and watch the mixture re-balance.
Notice that K does not change when you change the total amount C - it is a constant of the reaction at a given temperature, and only the concentrations rescale. This is a fixed demonstration concept (NOVA's interactive-visual study modality); the per-misconception, per-course generated version is the frontier work that follows.