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Basic 3 · Mathematics · Number Operations

Adding and Subtracting within 1000

In this topic you will add and subtract 3-digit numbers using the column method, including regrouping.

By the end of these notes you should be able to:

  • Add 3-digit numbers in columns
  • Subtract 3-digit numbers in columns
  • Regroup (carry and borrow) when needed

What you already know

  • Adding and subtracting to 100 (Basic 2)

Copy everything that has a blue line on its left into your notebook. The other parts are for reading.

Strand
Number
Sub-strand
Number Operations
Indicator
B3.1.2.1.1 – Use standard strategy or procedure to do addition or subtraction within 1000

Introduction

A school has 276 boys and 348 girls. How many pupils are there? We line up the numbers in columns and add.

1. Adding in Columns

Example

276 + 348 = ?

  1. Ones: 6 + 8 = 14. Write 4, carry 1 ten.
  2. Tens: 7 + 4 + 1 = 12. Write 2, carry 1 hundred.
  3. Hundreds: 2 + 3 + 1 = 6.

Answer: 276 + 348 = 624

Column addition
HTO
276
+348
=624
Column addition of 276 + 348 with carried digits shown
Figure 1: Adding with carrying Copy the sum and show the carried digits.Illustration: Sirbus

2. Subtracting in Columns

Example

532 − 178 = ?

  1. Ones: 2 − 8 cannot be done. Borrow 1 ten: 12 − 8 = 4.
  2. Tens: now 2 − 7 cannot be done. Borrow 1 hundred: 12 − 7 = 5.
  3. Hundreds: now 4 − 1 = 3.

Answer: 532 − 178 = 354

Tip: Check subtraction by adding: 354 + 178 = 532.

New words

column
A line of digits going down.
regroup
To change 10 ones into 1 ten, or 1 ten into 10 ones.
carry
To move a group of ten to the next column.
borrow
To take 1 from the next column to make 10.

Summary

  • Line up hundreds, tens and ones.
  • Start with the ones. Carry or borrow when needed.
  • Check subtraction by adding.

Class activity: Column Challenge

  1. Pick two 3-digit number cards.
  2. Add them in columns.
  3. Subtract the smaller from the larger.
  4. Check your partner's work.

You need: Raffia string, Measuring sticks, Assorted empty cups, Bottles, Bottle tops, Number cards

Exercises

Answer in your exercise book first, then check your answers.

  1. 1. 145 + 232 = ?

    • A. 377
    • B. 367
    • C. 387
    • D. 477
    Show answer
    Answer: 377No carrying needed.

  2. 2. 468 + 357 = ?

    • A. 825
    • B. 815
    • C. 725
    • D. 835
    Show answer
    Answer: 8258 + 7 = 15, 6 + 5 + 1 = 12, 4 + 3 + 1 = 8.

  3. 3. We start adding from the ones column.

    • A. True
    • B. False
    Show answer
    Answer: TrueOnes come first.

  4. 4. 603 − 158 = 455

    • A. True
    • B. False
    Show answer
    Answer: False603 − 158 = 445.

  5. 5. 300 + 450 = ______

    Show answer
    Answer: 7503 hundreds + 4 hundreds 5 tens.

  6. 6. A farmer had 640 yams. He sold 275. How many are left?

    Show answer
    Answer: 365 yams640 − 275 = 365.

Homework

  1. Make up two word problems using numbers up to 1000. Solve them in columns.