Shopping for Plants

Garden Delight sells marigolds in six-plant packs and twelve-pack flats. Each pack costs $1.00, and each flat costs $10.00. Green Thumb sells packs for $0.89 and flats for $6.50. Each of its packs holds four plants, and each flat has twelve packs. If Mr. Alvarez wants seventy-two plants, which shop has the better buy?

Extension:
How many more plants would Mr. Alvarez need for him to believe the other shop has a better buy?

(Source: Mathematics Teaching in the Middle School, May 1998)