For three it is 2:1
For four it is either 2:2 or 3:1
For five it is 3:2
…and “it has to be maximum”? Yes! That’s it!
Now that I know the selections cannot be pure…
YES! With the maximum, even if it is purely muddy waters, the user won’t learn anything new! (Well, no. That’s arguably VERY wrong, because if you know the number of wet mud, then it would be very much able to clean–) Okay, okay! Then, how about forcibly setting the number such that it always fails if the chosen number is equal to or under the wet mud?
Alright. Let’s do this again from the top.
Divide everyone into groups.
Number of groups has to be more than the number of wet mud. (Else all fails.)
Results may vary depending on the number of contaminated groups.
…YES! This is it!