Dividing a given quantity into given ratios

@lev888 Doesn't cup inherently mean we are talking about the standard unit size(236.8ml) .
if suppose, the word cup is not used in ratios , 1 cup water and 2 cups milk then also we know we are talking about same size .
Once we are talking about the same attribute, in this case the volume of a fluid, all that is important is that the “parts,” WHATEVER NAME THEY ARE GIVEN, represent multiples of the same thing.

Yes, it probably will be interpreted as the standardized measure called a “cup” in the U.S. That is entirely irrelevant. You could substitute “bucket,” which is not a standardized measure, and the ratio would be just as valid with respect to fluid volume, provided you use the same bucket. Or you could substitute liter. The unit is arbitrary and need not represent a standardized unit.

Where standardized measurements become essential to the meaning of a ratio is if not all the numbers in the ratio relate to the same attribute. If 2 cups of water to 3 ounces of flour is what is specified, then standardized units become essential. If you change either or both of the units, the ratio will change.
 
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