These sequence questions drive me up the wall. It's so unbelievably stupid that out of the near infinite functions that can produce a given list you should come

These sequence questions drive me up the wall. It's so unbelievably stupid that out of the near infinite functions that can produce a given list you should come up with the one the interviewer thought was the "obviously correct" one. The sequence in the example: 1, 11, 21, x ... with the information given then the X=31 is just as valid as anything that the interviewer came up with.