Jugs are not a standard size so the answer depends on the jug.
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There is 1 gallon of milk in the typical milk jug.
5 gallons worth
Fill 7 gal jug with water and then dump into 4 gal jug leaving 3 gals in the 7 gal jug; throw out the 4 gal jug of water and pour the 3 gals from the 7 gal jug into the 4 gal jug leaving nothing in the 7 gal jug and 3 gals in the 4 gal jug; fill the 7 gal jug with water and pour into 4 gal jug till full, leaving 6 gals in the 7 gal jug
6 gallons
Katie will need 6 gallons of ginger ale.