The original size of the bottle is 17.5 ounces.
The soap manufacturer has increased the size of his bottles. This means that 18.4 ounce bottle is bigger than the original size of the bottle. The bottle was increased by 5%, this means that the new bottle is 105% (100% + 5%) bigger than the initial size of the bottle.
To determine the original size of the bottle, this formula would be used:
New bottle size / total percentage increase in the bottle = initial size of the bottle
18.4 / 1.05 = 17.5 ounces
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