More detail in the protocol would be helpful. For example, do you get to repeatedly use the same bottle of the set? If so, I can do the following for a 20 trial set: Pick bottle 1. Now run through the other 19 bottles and guess for each that it is different from bottle 1. I'll be correct 10 out of 19 trials. This method generalizes in a fairly obvious fashion although it isn't clear to me if one is going to do n trials whether this is actually the optimal procedure for maximizing how often you are correct. I suspect that one can do better but it isn't clear to me how.