First find out how many ounces of milk James had to begin with by multiplying the number of gallons by the number of ounces per gallon: 3 gallons * 128 ounces/gallon = <<3*128=384>>384 oz
Now subtract the 13 ounces James drank from the initial amount of milk: 384 oz - 13 oz = <<384-13=371>>371 oz
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