In the laws of Ona'ah, three categories exist: less than one-sixth, more than one-sixth and exactly one-sixth.
Regarding exactly one-sixth, in the majority of cases, the exact one-sixth would not be an exact coinage; it would be an irrational number or multiple decimals not equating to an exact amount. Therefore, two options seem to exist:
One, it has to be exact and if not would not be applicable (in more than 60% of cases); or two, the halacha would allow rounding to the nearest amount. If this second case, would one be allowed to round up and round down? How much percentage difference would be allowed? If rounding, would one round to the the second decimal, or to the nearest whole number? etc.
Thanks for the consideration.
Benyomin Hoffman, Los Angeles, CA
The number is exact, as indicated in the cases in the Gemara. I assume you are correct that most cases would probably not be exact. However, there are many laws in the Torah that happen less frequently than exact Ona'ah, and there is more written about them (i.e. Ben Sorer u'Moreh). It is not a reason to say that Ona'ah is not exact.
All the best,