KS2 Algebra — Year 6 Guide to Missing Numbers & Equations

Algebra Isn’t as Scary as It Sounds

Remember those boxes in Year 1? “3 + □ = 7”. Your child knew the answer was 4. That was algebra. The only difference now is that the box has been replaced with a letter. 3 + a = 7. Same question. Same thinking.

KS2 algebra doesn’t involve anything complicated like quadratics or simultaneous equations. It’s about using what you already know — addition, subtraction, multiplication, division — and working backwards to find a missing value. If your child can work out how much change from a fiver, they can do this.

Finding Missing Numbers

The simplest type: one operation, one unknown.

The key skill here is inverse operations. If the equation uses addition, undo it with subtraction. If it uses multiplication, undo it with division. That’s the whole trick.

Always check by substituting back in. Does 27 + 15 = 42? Yes. Job done.

Two-Step Problems

This is where it gets a bit more interesting. Two operations instead of one.

Think of it like peeling an onion — undo the outer layer first, then the inner layer. The last thing that was done to x gets undone first.

A real stumbling block for some children is knowing which operation to undo first. The rule is simple: deal with addition/subtraction before multiplication/division. Work from the outside in.

Number Sequences

These come up on nearly every reasoning paper. Your child sees a sequence and has to find the rule, then work out missing terms.

Linear sequences (where you add or subtract the same amount each time) are the most common. But SATs sometimes throw in sequences that involve two operations:

Encourage your child to look at the gaps between numbers first. If the gaps are all the same, it’s a simple rule. If the gaps are growing, try doubling or squaring.

Using Formulae

Your child will need to substitute numbers into simple formulae. This sounds fancy but it’s really just swapping letters for numbers.

The most common mistake here is forgetting the order of operations — doing the multiplication before the addition inside the brackets. Remind your child: brackets first, always.

Algebra in the Reasoning Papers

Algebra questions on the reasoning papers are usually worth 1–2 marks. They’re typically a “find the value of” question:

Add 5 to both sides: 4n = 24. Divide by 4: n = 6. Two marks for about twenty seconds of work.

Sometimes they disguise algebra as a shape problem (“the perimeter is 34 cm, find the missing side”) or a pattern problem (“how many squares in pattern number 10?”). The method is the same: set up a simple equation and solve it.

Tips from Teachers

• Use inverse operations — This is the single most important skill. If your child can “undo” an operation, they can solve any KS2 equation.
• Always check by substituting back — Found x = 6? Plug it back into the original equation. Does it work? If yes, you’re golden.
• Draw a bar model if stuck — Bar models turn abstract equations into something visual. They’re brilliant for two-step problems.
• Don’t panic about letters — x, n, a — they’re just a placeholder for the number you need to find. Nothing more.
• Practise sequences separately — Spotting the rule is a different skill from solving equations. Give both some dedicated time.