12) A
The offer: 3 boxes for the price of 2
Price of 2 = $1.50 x 2 = $3.00
Since 3 boxes are sold at $3.00, price of 1 box = 3.00 ÷ 3 = $1.00
Price reduction = 1.50 – 1.00 = $0.50
Since the supermarket always makes 40% gross margin, they always pay the suppliers 60% of the cost of the cereal.
Price the supermarket paid before the offer = 0.60 x 1.50 = 90 c
Price the supermarket pays during the offer = 0.60 x 1.00 = 60 c
Difference = 90 – 60 = 30c