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Word Problems Within 20

A 40-minute Grade 1 lesson covering the four situations a word problem can describe — joining, taking from, putting together and comparing — and why the last of these is the one that catches everybody.

Word Problems Within 20 is a free 40-minute Grade 1 maths lesson plan for Common Core standard 1.OA.A.1 — solve story problems with adding and subtracting up to 20. 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.A.1

Grade 1 · Operations & Algebraic Thinking

1.OA.A.1

Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing.

Objective

Students will be able to solve addition and subtraction word problems within 20 by identifying the situation the story describes.

Students are successful when they can

  • Says which of the four situations a problem describes before solving it.
  • Draws a bar or a part-part-whole diagram that matches the story.
  • Solves a comparison problem without simply adding the two numbers.

What you need

  • Counters or cubes, 20 per pair
  • Blank part-part-whole mats
  • Story problem cards covering all four situations
  • The printable practice sheet linked at the end of this plan

The lesson

Warm-up — what happened?

5 min
  1. Read four short problems aloud. After each, ask only what happened — not the answer.
  2. Collect the four descriptions: some joined, some left, two groups together, one group compared with another.

Teach — four stories, one diagram

12 min
  1. Take a joining problem — 7 birds, 5 more land. Build it on a part-part-whole mat: two parts known, the whole unknown.
  2. Take a taking-from problem — 12 birds, 5 fly off. Same mat: the whole and one part known.
  3. Point out that both used one mat. The mat does not care which story it was; it cares which piece is missing.
  4. Now a comparison: Ana has 12 stickers, Ben has 5. How many more does Ana have? Draw two bars, one under the other.
  5. The difference is the bit sticking out. Nothing joined and nothing left, which is why the mat feels wrong here and the bars do not.
“Find the missing piece, not the operation. If you know both parts you add; if you know the whole and one part you subtract. The story tells you which you have.”

Guided practice — diagram before answer

11 min
  1. Pairs take a story card, draw the diagram, and only then write the equation.
  2. Rule for the session: no number sentence until the diagram is drawn.
  3. Circulate and ask which piece is missing. A pair who can point to it will get the operation right without being told.

Independent practice

9 min
  1. Students work a mixed problems page.
  2. Mark the diagrams, not just the answers. A right answer with no diagram on a comparison problem is usually a coincidence.

Close

3 min
  1. Read one comparison problem and ask only for the diagram.
  2. Two bars with the difference marked is the picture to leave them with.

What goes wrong, and why

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

Adds the two numbers in 'Ana has 12, Ben has 5, how many more does Ana have?'

Why: The word 'more' has been learned as a signal to add. On joining problems that works every time, so the strategy has a long record of success behind it.

Fix: Draw the two bars. There is no joining anywhere in the picture, and the question is clearly about the gap. Never teach the keyword in the first place.

Subtracts the smaller from the larger regardless of the story.

Why: A fallback rule that produces a plausible number, adopted because the story was not understood and something had to be written.

Fix: Require the retelling before the calculation. A child who can retell the story in their own words rarely picks the wrong operation.

Cannot start a problem where the change is unknown — 'Ben had 6, got some more, now has 13'.

Why: The story is being followed forwards, and the missing piece is in the middle rather than at the end where it has always been.

Fix: Fill the part-part-whole mat with what is known first — the whole is 13, one part is 6. The gap appears on the mat and the child can see what to find.

If they are not there yet

  • Work within 10 and with joining and taking-from only, adding comparison once the diagram habit is established.
  • Give the mat with the known numbers already filled in, so the work is identifying the gap rather than reading and recording at once.

If they finish early

  • Give problems where the start is unknown, which cannot be solved by following the story forwards.
  • Ask the child to write a comparison problem with the answer 4.
  • Give a two-step problem: some joined and then some left.

Exit ticket

Sam has 14 marbles. Kim has 6. How many more marbles does Sam have?

What to look for: 8, ideally with two bars drawn. An answer of 20 is the keyword strategy firing — that pupil needs the bar model, not more practice at subtraction.

Printable practice for 1.OA.A.1

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

Questions about Word Problems Within 20

How long does the Word Problems Within 20 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 Word Problems Within 20 teach?

CCSS.MATH.CONTENT.1.OA.A.1 — 1.OA.A.1, Operations & Algebraic Thinking for Grade 1: “Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing.” In plain terms, solve story problems with adding and subtracting up to 20.

What do I need to teach this lesson?

Counters or cubes, 20 per pair, blank part-part-whole mats, story problem cards covering all four situations 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: adds the two numbers in 'Ana has 12, Ben has 5, how many more does Ana have?'; subtracts the smaller from the larger regardless of the story and cannot start a problem where the change is unknown — 'Ben had 6, got some more, now has 13'. 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: “Sam has 14 marbles. Kim has 6. How many more marbles does Sam have?.” 8, ideally with two bars drawn. An answer of 20 is the keyword strategy firing — that pupil needs the bar model, not more practice at subtraction. 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 Word Problems Within 20?

Yes — 2 printable 1.OA.A.1 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 Word Problems Within 20 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.A.1 of the Common Core State Standards for mathematics.