NS-006

Find the next number: 9, 21, 24, 36, 39, ?

Rule: The pattern alternates: +12, +3, +12, +3, ... so the next step is +12.

So the next number is 51.

• Picking the first pattern you see without checking it against all rows/columns.
• Ignoring the simplest rule (counts, rotations, alternations) and jumping to a complex story.
• Not testing your rule on the middle row/column (where many puzzles “break” bad rules).