Number series — find the rule

A number series is a list of numbers that follows a hidden rule. Your job: spot the rule, then predict the next term.

The most common rules:

• +/− a constant: 3, 7, 11, 15, ... (rule: +4)
• ×/÷ a constant: 2, 6, 18, 54, ... (rule: ×3)
• Squares/cubes: 1, 4, 9, 16, ... (rule: n², so next is 25)
• Skip differences: 2, 3, 5, 8, 12, ... (differences 1, 2, 3, 4 — next is 17)