What statistic does the author use to illustrate the growth of the fast-food industry?

The statistic that highlights the growth of the fast-food industry is the number of fast-food outlets and sales growth. This metric is significant because it directly reflects the expansion and popularity of fast-food chains over time. By measuring how many new outlets have opened and the corresponding sales figures, one can gauge the overall health and trajectory of the industry. This data points to consumer demand, market trends, and the economic impact of fast food on society. It showcases not only how widespread and accessible fast food has become but also illustrates its financial success, which is essential for understanding its growth as a cultural and economic phenomenon.