Multiple choice general knowledge math & puzzles Odd one out 213 111 121 222 333 201 303 303 213 201 121 Reveal answer Fill a bubble to check yourself D Correct answer Explanation In the set, all numbers except 121 have a digit sum that is a multiple of 3 or follow a specific pattern of external digits summing to the middle (except 121 is a perfect square). More simply, 121 is the only perfect square (11^2) in the list.