Did you mean 1 and 1/4 gallons? Or did you mean 1 gallon + 4 gallons? I'll answer both ways:
A gallon has 128 fluid ounces. So if the question is 1 and 1/4 gallons, you divide the 128 ounces by 4, which is 32 ounces. You then add 32 + 128, which = 160 fluid ounces.
If your question is how many fluid ounces in 1 gallon + 4 gallons, you simply multiply 5 x 128, which = 640 fluid ounces.
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