Convert 15 gallons to 750 ml:

1 gallon = 3785.411784 ml

15 gallons = 15 * 3785.411784 ml = 56781.17676 ml

Now divide the total volume in ml by the volume of each bottle in ml:

56781.17676 ml / 750 ml = 75.7 bottles

So, there are approximately 75.7 bottles of 750ml in 15 gallons.