By the use of proportion, it would last him for 6.67 days.
From the question, we already had the information that the evil Barty Crouch Jr. had to drink at least 4 fl. oz. of Polyjuice Potion every hour in order to maintain his disguise as Mad Dog Moody.
Now;
If 128 fl. oz. = 1 gallon
x = 5 gallons
x = 640 fl. oz
If he takes 4 fl. oz. in 1 hour
He would take640 fl. oz in x hours
x = 640 fl. oz * 1 hour/4 fl. oz
x = 160 hours
1 day is made up of 24 hours
x days is made up of 160 hours
x = 6.67 days
Learn more about proportion:https://brainly.com/question/2548537
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