Incomes today are higher than they were 30 years ago, and 30 years ago single people spent a higher percentage of their income on food than people do today. Therefore, incomes have increased faster than food prices over the past 30 years.

The argument moves from a statement about how much people spend on food over time to a conclusion that the change in spending is because of a change in the cost of food items. It doesn't, however, explicitly state that single people kept buying the same exact food over those years. It’s still possible that the reason single people spend less of their income on food today is because they’re eating less or because they’re eating cheaper foods. We’re therefore looking for some assumption that helps justify the conclusion by ruling out possible alternative explanations of the trend.