Primary 2 Maths | Three-Digit Addition with Carrying Guide

Primary 2 Maths | Three-Digit Addition with Carrying Guide | Align, Carry, Check

2026-11-09

Tutor lesson-prep guides · Primary Maths

A child writes 315 for 268 plus 157 because the carried 1 never reached the tens column. This lesson drills one habit: after each column, ask whether it reached ten.

Objectives — Students can: (1) set out a three-digit addition in columns with ones, tens and hundreds aligned; (2) carry 1 to the next column whenever a column reaches ten, writing the 1 above the next column; (3) check the answer by swapping the addends or subtracting one addend from the sum.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Addition within 20	What is 8 plus 7	15
Place value	Is the 6 in 268 in the tens or ones place	Tens
Column form	Which side do ones go in a vertical sum	Far right

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Diagnostic	Let them compute 268 plus 157 and collect the wrong answers	Try it once
8-18 min	Alignment	Ones, tens, hundreds on three rows; show what misalignment does	Rewrite the sum
18-32 min	Carrying	Ones: 8 plus 7 is 15, write 5, carry 1	Say each carry aloud
32-44 min	Double carry	486 plus 275 carries in the tens too	Finish then compare
44-54 min	Checking	Use 425 minus 157, or swap the addends	Both checks once
54-60 min	Exit check	Three questions collected and marked	Work independently

03 Teaching script (read it out)

Scene 1: Align first, then add

A vertical sum has one rule: the same place value goes in the same row. Ones under ones, tens under tens, hundreds under hundreds, with ones at the far right.
Why align? The 6 in 268 is six tens and the 7 in 157 is seven ones; adding them would otherwise pretend they are thirteen tens. Children who misalign usually understand addition fine; they are treating digits as a string of symbols.

Students do: write 268 plus 157 in columns and label each row ones, tens, hundreds.

Scene 2: Carry when a column reaches ten, and write the 1

Each column holds a single digit from 0 to 9, so ten moves one unit to the next column. For 268 plus 157: ones 8 plus 7 is 15, write 5 and carry 1; tens 6 plus 5 plus the carried 1 is 12, write 2 and carry 1; hundreds 2 plus 1 plus 1 is 4. The answer is 425.
The critical move is writing that small 1 above the next column instead of holding it in the head. A double carry such as 486 plus 275 equals 761 is the same move twice: tens 8 plus 7 plus 1 is 16, so write 6 and carry 1 again.

Students say: what the carried 1 actually is, and why it joins the next column rather than staying put.

Scene 3: Check it: add the other way, or subtract back

There are two checks. Swap the addends: 157 plus 268 must also give 425. Or subtract one addend from the sum: 425 minus 157 equals 268, and getting back the original number means the addition was right.
Add a sanity check too: the sum of two three-digit numbers is at most 1998, and it must be larger than either addend. A sum of 425 that is smaller than 268 cannot be right.

Students do: check 907 equals 709 plus 198 by subtraction, and explain why the tens digit is 0.

04 Question types and worked steps

Type	Example	Steps	Answer
Column addition	Three digits plus three digits	Align, add each column, carry at ten	Write the carried 1 on the sum
Double carry	Both tens and hundreds carry	Add the carry before the digits	Never skip the carry
Missing number	268 plus what equals 425	Subtract the known addend from the sum	Check the digit count
Word problem	Two quantities combined	Write the sum, compute in columns, state units	Answer sentence needs units

Sanity check (teach this): Sanity check: three-digit plus three-digit is at most 1998, never a much larger number; the sum is larger than each addend; at most two carries into the thousands; 268 plus 157 is 425, not 315 or 415.

05 Independent practice (3 questions)

Question	Answer	What to watch
Alignment	Complete three half-written column sums	Ones lined up
Single carry	268 plus 157, showing each carry	The carried 1 is written
Double carry	486 plus 275 and 709 plus 198	The zero in the tens is not dropped
Checking	Check two answers by subtraction	Check direction not reversed

06 Common mistakes and what to say

Mistake	Student writes	Say this
Dropping the carry	268 plus 157 gives 315	The ones column carried 1 into the tens; did you add that 1 in the tens column?
Misaligned columns	157 shifted one column left	The 7 is seven ones and must sit under the 8; which row did you put it in?
Starting from the left	Adding hundreds first	We start at the ones because carries move left into the next column.
No units in word problems	Answering just 425	425 of what? Books? People? Write the unit.

07 Homework (paper, section and question numbers)

Primary maths chapter index — All chapters by level
Teaching guide index — Same structure, other subjects
Redo this chapter — Questions 1 to 4 again, marking each carry and checking by subtraction

08 Exit check (last 5 minutes)

Question	Answer	Decision
268 plus 157	425	315 means a dropped carry, back to act two
Most carries for three-digit plus three-digit	Two	One means repeat the double carry
How to check	Swap addends or subtract an addend from the sum	Just doing it again means act three

09 Tutor FAQ

Can they use mental arithmetic instead?

Yes, once the written form is correct. Columns make the carry visible, and fluent children move on by themselves.

Why write the carried 1?

It moves working memory onto paper. A Primary 2 child holding three digits already struggles; writing the 1 removes one thing to remember.

Does this work for four digits?

Identically, with one more row. Alignment and carrying at ten are the same method used for larger numbers later.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.