Zerdly

Number Patterns

Spot the rule, then run it backwards. It starts with counting patterns and finishes with equations — which turn out to be the same puzzle wearing different clothes.

If the word algebra puts you off, ignore it. You have been doing this since you first filled in a missing number.

No account, and the maths stays in your browser — more

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Algebra is a missing number with a name

3, 6, 9, ?, 15 and 3 × ? = 12 ask the same thing: what does the rule do when you run it backwards? Later on the ? gets called n or x, and nothing else changes. That is the entire trick.

Three things to watch for

  • The sign you can see is not the sum you do. In 8 + ? = 20 the plus is part of the puzzle, not an instruction. To find the gap you take away.
  • Undo the last thing first. For 3 × ? + 4 = 19, the 4 was added at the end, so take it off before you divide. Doing it the other way round gets a different, wrong answer.
  • Term 1 is the start, not the first step. Getting to term 6 takes five steps, not six. It is the most common slip once patterns get a term number.

School levels

School levels are a guide only. They show where a skill is usually taught in each country and are not taken from, aligned to, or endorsed by any curriculum authority. Your school may cover it earlier or later.