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Basic 3 · Mathematics · Number Operations

Equal and Not Equal, Missing Numbers

In this topic you will use = and ≠ and find missing numbers in number sentences.

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

  • Decide if two number sentences are equal
  • Use = and ≠ correctly
  • Find missing numbers to balance sentences

What you already know

  • Equal and not equal (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
B3.1.2.2.1 – Use the concept of "equal to" and "not equal to"

Introduction

A see-saw is balanced when both sides weigh the same. A number sentence with = is balanced too.

1. Balanced Sentences

250 + 150 = 300 + 100 because both sides are 400.

250 + 150 ≠ 500 because 400 is not 500.

A balanced scale with 250 + 150 on one side and 300 + 100 on the other
Figure 1: A balanced number sentence Draw a balance for 120 + 80 = 100 + 100.Illustration: Sirbus

2. Finding Missing Numbers

Example

45 + ___ = 60 + 15

  1. Right side: 60 + 15 = 75
  2. 45 + ? = 75
  3. 75 − 45 = 30

Answer: The missing number is 30.

Example

___ − 120 = 380

  1. ? − 120 = 380
  2. 380 + 120 = 500

Answer: The missing number is 500.

New words

balanced
Equal on both sides.
equal
The same value (=).
not equal
Different value (≠).
missing number
An unknown number we must find.

Summary

  • = means both sides have the same value.
  • ≠ means the values are different.
  • Use adding or subtracting to find missing numbers.

Class activity: Balance It

  1. Write a number sentence with a missing number.
  2. Swap with another group.
  3. Find the missing number and check both sides.

You need: Bottle tops, Bundles of sticks, Exercise books

Exercises

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

  1. 1. Which is true?

    • A. 200 + 300 = 500
    • B. 200 + 300 = 600
    • C. 200 + 300 ≠ 500
    • D. 200 = 300
    Show answer
    Answer: 200 + 300 = 500Both sides are 500.

  2. 2. 70 + ___ = 100 + 20

    • A. 50
    • B. 30
    • C. 120
    • D. 40
    Show answer
    Answer: 50100 + 20 = 120; 120 − 70 = 50.

  3. 3. 500 − 100 = 200 + 200

    • A. True
    • B. False
    Show answer
    Answer: TrueBoth sides are 400.

  4. 4. 360 + 40 ≠ 400

    • A. True
    • B. False
    Show answer
    Answer: False360 + 40 = 400.

  5. 5. 15 + 25 = 10 + ______

    Show answer
    Answer: 30Both sides must be 40.

  6. 6. Is 125 + 75 equal to 250 − 50? Show why.

    Show answer
    Answer: 125 + 75 = 200 and 250 − 50 = 200, so they are equal.Both sides are 200.

Homework

  1. Write three number sentences with = and two with ≠.