Yes, Mai is correct.
According to the recipe, 5 scoops of mix are used for every 4 cups of water. We can express this as a ratio:
5 scoops mix : 4 cups water
If we simplify this ratio by dividing both sides by 4, we get:
5/4 scoops mix : 1 cup water
So for every 1 cup of water, we need 5/4 scoops of mix, or 1.25 scoops. This means that for any amount of lemonade we make, we will always need one more scoop of mix than the number of cups of water.