The last question has emphisis on sticker. Look closely at the sticker. Instead of putting 20% off they put 二割引 sticker, it means 20% off. Some supermarket has the literal 20% off sticker but as you can see they have that kind of sticker, too. 2割引 = 20% off 500 × .20 =100 500 - 100 = 400円 Trickyですね.