(1)
20瓶以下,选甲商场
设消费者买x瓶(x≥20),则若选甲商场需花1.2×0.9×x = 1.08x
若选乙商场则需花：20×1.2+(x−20)×1.2×0.8 = 4.8+0.96x
4.8+0.96x−1.08x = 4.8−0.12x
不难看出当x0,当x = 40时,4.8−0.12x = 0,当x>40时,4.8−0.12x