At the original prices, the consumer consumes 15 quarts of Pepsi and 5 quarts of Coke at the utility-maximizing bundle A. When the price of Pepsi doubles, the consumer’s budget constraint rotates inward from BC₁ to BC₂. At the new optimal consumption bundle B, the consumer decreases his consumption of Pepsi from 15 to 5 quarts and increases his consumption of Coke from 5 to 10 quarts. Since the quantity of Coke demanded rose while the price of Pepsi rose, Coke and Pepsi are considered substitutes.