There should be pairs available which mean that the letters should be divided into equal amounts. PAC and ETS can be the two pairs because the letter that will be between them is K. It states that the letter separating them should be consistent whether the word will be forward or backward.
It cannot be one because the word has to be separated in pairs. It cannot be three because the letters between them will not be consistent. It also cannot be four because one letter will not have any pair. More than four is just not possible at this time.
