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Subtraction as a Missing Part

A 40-minute Grade 1 lesson that reframes 13 − 8 as 'eight and what makes thirteen', which is both easier to work out and the reason fact families exist.

Subtraction as a Missing Part is a free 40-minute Grade 1 maths lesson plan for Common Core standard 1.OA.B.4 — see subtraction as finding the missing part. It is timed across 5 sections, gives the exact wording to use where the wording carries the mathematics, names 3 misconceptions with the teaching move that addresses each, and ends with an exit ticket and a printable worksheet with its answer key.

Grade:
Grade 1
Length:
40 minutes
Standard:
1.OA.B.4

Grade 1 · Operations & Algebraic Thinking

1.OA.B.4

Understand subtraction as an unknown-addend problem.

Objective

Students will be able to solve a subtraction within 20 by finding the unknown addend in the matching addition.

Students are successful when they can

  • Rewrites 13 − 8 as 8 + ___ = 13.
  • Uses a known addition fact to answer a subtraction.
  • Writes the four facts of a fact family from one part-part-whole picture.

What you need

  • Part-part-whole mats
  • Cubes, 20 per pair
  • Fact family triangles
  • The printable practice sheet linked at the end of this plan

The lesson

Warm-up — how many hidden?

5 min
  1. Show 12 cubes, then cover some and reveal the rest. Ask how many are hidden.
  2. Do six rounds. Nobody will call this subtraction, and everybody will be doing it.

Teach — the same picture, two questions

12 min
  1. Fill a part-part-whole mat: whole 13, one part 8, other part hidden.
  2. Ask the question two ways — 'thirteen take away eight' and 'eight and how many more makes thirteen' — and point out the mat did not change.
  3. Write 13 − 8 = ___ and 8 + ___ = 13 side by side.
  4. Ask which is easier to work out. Counting on from 8 to 13 is five steps; counting back from 13 is five steps too, but backwards, and backwards is where the errors live.
  5. Build the full fact family: 8 + 5, 5 + 8, 13 − 8, 13 − 5. One picture, four sentences.
“Subtraction is not a different thing from addition. It is the same three numbers with a different piece missing.”

Guided practice — flip it

11 min
  1. Pairs take a subtraction card and write the matching addition with a blank before solving.
  2. The partner checks the addition matches the subtraction before either answers it.
  3. Then write all four facts of the family.

Independent practice

9 min
  1. Students complete a think-addition page.
  2. Watch for pupils counting back on their fingers. The answer will be right; the strategy is the thing being taught.

Close

3 min
  1. Give 15 − 9 and ask for the addition that answers it.
  2. 9 + 6 = 15, and the subtraction is done. Stop on that.

What goes wrong, and why

Each of these is a reasoning error rather than carelessness. The fix addresses the reasoning.

Writes 13 − 8 as 8 − 13 when rearranging.

Why: The numbers have been moved around as symbols without reference to which is the whole. Turning things round worked for addition, so it is being tried here.

Fix: Put the whole at the top of the mat every time. Thirteen is the whole; it cannot become a part because there is more of it than either piece.

Counts back for every subtraction, including 13 − 2 and 13 − 11.

Why: One reliable method has been learned and is applied uniformly. Choosing a method by looking at the numbers first is a new demand.

Fix: Give 13 − 11 and time it against a partner counting up from 11. The gap in effort makes the case; a rule about when to use which would not.

Writes only three facts in the fact family, missing one subtraction.

Why: Both subtractions start with the same whole, so the second one looks like a repeat of the first.

Fix: Cover each part in turn on the mat. Two parts, two things that can be hidden, two subtractions. The mat generates the fourth fact rather than a rule.

If they are not there yet

  • Work within 10 first, where the addition facts are already known and the reframing is the only new thing.
  • Keep the cubes and the physical covering. The hidden part is what makes the unknown addend concrete.

If they finish early

  • Solve a subtraction with the start unknown: ___ − 6 = 9.
  • Use fact families to answer a subtraction the pupil has never met, from an addition they know.
  • Extend to numbers within 100 where the same reframing makes 62 − 58 far easier than it looks.

Exit ticket

Write the addition that helps you solve 16 − 9, then answer it.

What to look for: 9 + 7 = 16 and the answer 7. A pupil who wrote only the answer may have counted back — ask them to show the addition tomorrow before accepting the strategy is there.

Printable practice for 1.OA.B.4

Free to print and copy, answer key included. Each sheet can generate a fresh set of problems for a retake.

Questions about Subtraction as a Missing Part

How long does the Subtraction as a Missing Part lesson take?

40 minutes, split across 5 timed sections: warm-up 5 min, teach 12 min, guided practice 11 min, independent practice 9 min, close 3 min. The minutes are stated per section and add up to the stated length, so the plan can be cut or extended at a section boundary rather than abandoned halfway.

Which standard does Subtraction as a Missing Part teach?

CCSS.MATH.CONTENT.1.OA.B.4 — 1.OA.B.4, Operations & Algebraic Thinking for Grade 1: “Understand subtraction as an unknown-addend problem.” In plain terms, see subtraction as finding the missing part.

What do I need to teach this lesson?

Part-part-whole mats, cubes, 20 per pair, fact family triangles and the printable practice sheet linked at the end of this plan. Everything listed is either ordinary classroom equipment or a free printable from this site — there is nothing to buy and nothing to prepare beyond photocopying.

What mistakes should I expect in this lesson?

Three, each a reasoning error rather than carelessness: writes 13 − 8 as 8 − 13 when rearranging; counts back for every subtraction, including 13 − 2 and 13 − 11 and writes only three facts in the fact family, missing one subtraction. The plan gives the thinking behind each one and the teaching move that addresses the thinking rather than the symptom.

How do I know whether the lesson worked?

The exit ticket asks: “Write the addition that helps you solve 16 − 9, then answer it.” 9 + 7 = 16 and the answer 7. A pupil who wrote only the answer may have counted back — ask them to show the addition tomorrow before accepting the strategy is there. That tells you whether the class needs a reteach or a thirty-second conversation before you plan tomorrow.

Is there a worksheet to go with Subtraction as a Missing Part?

Yes — 2 printable 1.OA.B.4 worksheets, linked at the end of the plan and free like the rest of the site. Each one comes with an answer key and 25 versions, so the practice after the lesson can be a different paper on every desk.

Is Subtraction as a Missing Part free to use?

Yes. The plan, its printables and their answer keys are free to read, print and photocopy for your own class, with no account and no email capture. It was last reviewed on 5 September 2026. Please do not resell it or republish it as your own.

Last reviewed 2026-09-05. Aligned to 1.OA.B.4 of the Common Core State Standards for mathematics.