If one standard water bottle holds 16 fluid ounces (about 0.5 litre), how many such bottles of water make up 1 US gallon?

Introduction / Context:
In day to day life, people often talk about drinking a certain number of bottles of water per day, while health guides and science based discussions may use units like litres or gallons. To move between these descriptions, it is helpful to know how many standard bottles of water add up to one US gallon. This question checks your ability to use basic unit conversion between fluid ounces and gallons and to apply that conversion to a familiar bottle size.

Given Data / Assumptions:
- We are using the US gallon, which is defined as 128 US fluid ounces.
- One standard water bottle is taken as 16 US fluid ounces, which is roughly 0.5 litre.
- We assume bottles are completely filled to their labelled capacity.
- We ignore small manufacturing tolerances and rounding errors.

Concept / Approach:
The core idea is that the total volume in gallons must equal the sum of the volumes of the individual bottles. A US gallon contains 128 fluid ounces. If each bottle holds 16 fluid ounces, then the number of bottles needed is simply the total ounces in one gallon divided by the ounces per bottle. This is a straightforward ratio and proportion calculation in basic arithmetic and unit conversion.

Step-by-Step Solution:
Step 1: Write the definition of a US gallon in fluid ounces: 1 gallon = 128 fl oz. Step 2: Note the capacity of one bottle: 1 bottle = 16 fl oz. Step 3: Set up the calculation for number of bottles: number of bottles = total volume in ounces / volume per bottle. Step 4: Substitute the values: number of bottles = 128 / 16. Step 5: Perform the division: 128 / 16 = 8. Step 6: Conclude that one US gallon is equal to the volume of 8 bottles of 16 fluid ounces each.

Verification / Alternative check:
You can check the answer in reverse. If you take 8 bottles of 16 fluid ounces, the total volume is 8 * 16 = 128 fluid ounces. Since 128 fluid ounces is exactly one US gallon by definition, the reverse calculation confirms that 8 such bottles match one gallon. This symmetrical check gives confidence that the arithmetic and the logic of the conversion are both correct.

Why Other Options Are Wrong:
16 bottles of 16 fl oz each would give 16 * 16 = 256 fl oz, which corresponds to 2 US gallons, not 1.
4 bottles of 16 fl oz each give 64 fl oz, which is only half a gallon.
2 bottles of 16 fl oz each provide 32 fl oz, which is one quarter of a gallon.

Common Pitfalls:
A common mistake is to confuse litres and fluid ounces or to forget whether the gallon being used is the US gallon or the imperial gallon used in some other countries. Another error is to divide in the wrong direction, for example 16 / 128 instead of 128 / 16, which gives a fraction instead of the correct whole number of bottles. Carefully keeping track of units and always writing the ratio as total volume divided by volume per unit helps avoid these errors.

Final Answer:
One US gallon of water is equal to the volume of 8 bottles of 16 fluid ounces each.