Adding and subtracting tens mentally
Add and subtract a two-digit number and tens mentally and using concrete/pictorial representations
Lesson
Set out your lesson
“Adding and subtracting tens mentally means changing a two-digit number by a multiple of ten without recounting every one. 45 + 30 changes only the tens digit (4 becomes 7) because three tens were added; the ones stay 5. 82 − 40 removes four tens; the ones stay 2. We will build one model, then try mental answers with a quick check.”
Bring to the table
- About 40 large counters or base-ten style bundles (ten sticks made from taped straws or drawn tens) — parent keeps tiny pieces away from mouths
- Paper and pencil
Extra for the adult
Worked example. 45 + 30 = 75 because three tens join the four tens while the five ones stay. 82 − 40 = 42 because four tens leave and the two ones stay. The ones digit does not change when you add or subtract whole tens.
Optional challenge. For Adding and subtracting tens mentally, solve 68 + 50 and 105 − 30 and explain the tens-digit change near or past 100. Optional: make one short money or page-number story that adds or subtracts tens only.
What to notice. Keep one parent note: equations tried; answers; whether a tens model was used; the child’s words about which digit changed; support used. A photo of base-ten pieces alone cannot record the mental explanation. This session does not establish fluency for every tens jump.
Share and support
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Teaching approaches
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1 approach
Adding and subtracting tens mentally means changing a two-digit number by a multiple of ten without recounting every one. 45 + 30 changes only the tens digit (4 becomes 7) because three tens were added; the ones stay 5. 82 − 40 removes four tens; the ones stay 2. We will build one model, then try mental answers with a quick check.
Spend about 12 minutes adding and subtracting multiples of ten from two-digit numbers, first with tens bundles or drawings, then mentally, explaining which digit changes.
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Learning resources
Extra practice links — they do not replace Gakuva’s teaching approach.
The Math Learning Center · Practice link — not the full lesson
Number Pieces
Before you open · source notes
Adult-guided base-ten pieces for adding or subtracting tens mentally. Build a two-digit number, add/remove ten-rods, and say the new total.
Free · no login
Before you openSave before-and-after snapshots of a tens change (for example 34→54) and a parent note of the mental “+10/−10” statement.
Opening this link does not save learning or assess mastery.
Source, access and review notes
Unknown — link only
Toy Theater · Practice link — not the full lesson
Base Ten Blocks
Before you open · source notes
Virtual tens and ones for modeling ten more or ten less without counting up by ones.
Free · no login · ads
Before you openSave a screenshot of tens rods changing by one rod and a parent note of the starting and ending totals.
Opening this link does not save learning or assess mastery.
Source, access and review notes
Unknown — link only
Topmarks · Practice link — not the full lesson
Place Value Charts
Before you open · source notes
Interactive place-value chart for seeing how the tens digit changes when adding or subtracting tens.
Free · no login · ads
Before you openSave a chart screenshot showing a tens-digit change and a parent note explaining which place changed.
Opening this link does not save learning or assess mastery.
Source, access and review notes
Unknown — link only
Check understanding
- Calculate 45 + 30 = 75 mentally
- Calculate 82 − 40 = 42 mentally
- Explain that only the tens digit changes when adding/subtracting tens
“Can your child quickly work out '54 + 30' or '78 − 20' in their head — knowing that only the tens digit changes?”
Curriculum record
- Type
- Procedural
- Subject
- Mathematics
- Domain
- Addition & Subtraction
- Age range
- Ages 6–9
Standards