Historically, Starbucks Coffee has generated 50% of its revenue from sit-in customers, i.e. customers who spend 30 minutes or more in the coffee shop, and the remaining 50% from take-out customers, who buy and eat food outside. Since take-out customers are much more profitable and it is extremely difficult to attract new customers, Starbucks has started an advertising campaign in which it is creating new convenient offerings suitable for take-out customers. Because of this campaign, Starbucks' daily revenue has increased. Therefore, the average ticket price of take-out customer must have gone up since the total number of sit-in customers has remained unchanged.
What is the assumption that the author makes while drawing his conclusion?