NS-015

Find the next number: 1, 5, 9, 13, 17, ?

Rule: The pattern is an arithmetic sequence: add 4 each step.

So the next number is 21.

• 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).