The phrase "twice as much lemonade" tells you that if Mark sold 2 gallons and Regan sold twice as much as Mark, then Regan sold 2 * 2 which is 4 gallons, right? It's the same with the "2/3 as much". But since 2/3 is less than 1, the amount after you multiply some number by 2/3 is going to be less than the original number.
If Mark sold 2 7/8 gallons and Regan sold 2/3 as much, then Regan sold
I teach my algebra students to change that mixed fraction into an improper to make the multiplication easier: