The constant difference strategy is one of those subtraction methods that feels like magic to students once it clicks — and it takes a little of the fear out of subtracting larger numbers. If you teach Stage 2 maths, here is a clear, classroom-ready way to introduce it.
What is the constant difference strategy?
Constant difference works on a simple idea: if you shift both numbers in a subtraction by the same amount, the difference between them stays exactly the same. So a tricky calculation like 83 − 29 becomes a much friendlier 84 − 30 = 54. Students are not changing the answer — they are changing the question into one that is easier to solve.
Why it is worth teaching in Stage 2
Year 3 and Year 4 is exactly the right time to build flexible mental strategies, before students lock in to the formal algorithm for everything. Constant difference strengthens number sense, encourages students to look at a calculation before diving in, and gives them a reliable tool for subtracting near multiples of ten.
Teaching it explicitly: I do, we do, you do
Like most strategies, this lands best with explicit teaching and plenty of modelling.
- I do: Model a calculation like 83 − 29 on a number line, showing how shifting both numbers up by one keeps the gap between them identical.
- We do: Work through two or three together, thinking aloud about which adjustment makes the numbers friendliest.
- You do: Let students practise independently, then reflect on when the strategy is most useful.
Hands-on ways to practise
- Use a physical or drawn number line so students can see the difference staying constant as both numbers slide along.
- Give students subtraction problems and ask them to spot which ones are good candidates for constant difference (near multiples of ten are ideal).
- Bring in money contexts — adjusting to the nearest dollar makes the strategy feel purposeful and real.
Common misconceptions to watch for
The most frequent slip is students shifting the numbers in opposite directions, or adjusting only one number. Anchoring the strategy on a number line, where the gap is visible, helps students self-correct before it becomes a habit.
A ready-to-teach option
If you would like the modelling, guided practice and word-problem slides already prepared, our Constant Difference Subtraction lesson slides walk your class through the whole sequence — I Do, We Do and You Do, challenge tasks and reflection — ready to display on the board.
However you teach it, the goal is the same: students who pause, look at the numbers, and choose a smarter path to the answer.