SHS DirectorySHS MatchLesson PlansGES Promotion Past QuestionsStudent Lesson NotesBlogs

Basic 3 · Mathematics · Number Operations

Subtracting in Your Head

In this topic you will use quick strategies to subtract numbers within 100 in your head.

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

  • Subtract by counting back in tens and ones
  • Subtract by counting up to find the difference
  • Use round and adjust to subtract

What you already know

  • Adding in your head

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
B3.1.2.3.2 – Use strategies to mentally subtract whole numbers within 100

Introduction

You have GH₵50. A book costs GH₵35. How much change will you get? Count up from 35 to 50 in your head.

1. Strategies for Subtracting

Mental subtraction strategies
StrategyExampleHow
count back tens then ones85 − 3285 − 30 = 55, then 55 − 2 = 53
count up (find the difference)50 − 3535 + 5 = 40, 40 + 10 = 50; 5 + 10 = 15
round and adjust72 − 1972 − 20 = 52, then add 1 = 53
use addition facts16 − 99 + 7 = 16, so 7
Number line counting up from 35 to 50
Figure 1: Counting up to find the difference Draw counting up from 27 to 40.Illustration: Sirbus

Example

64 − 27 in your head.

  1. Subtract tens: 64 − 20 = 44
  2. Subtract ones: 44 − 7 = 37

Answer: 64 − 27 = 37

Tip: Counting up is the way shopkeepers give change.

New words

difference
The answer to a subtraction.
count back
To subtract by counting down.
count up
To find a difference by counting from the smaller number.
adjust
To change a little to fix the answer.

Summary

  • Count back in tens then ones.
  • Count up to find the difference.
  • Round and adjust: 72 − 19 = 72 − 20 + 1.

Class activity: Change Game

  1. One partner is a shopkeeper; the other buys items up to GH₵100.
  2. The buyer pays with a GH₵100 note.
  3. The shopkeeper counts up to give change in their head.

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

Exercises

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

  1. 1. 90 − 30 = ?

    • A. 60
    • B. 50
    • C. 70
    • D. 120
    Show answer
    Answer: 609 tens − 3 tens = 6 tens.

  2. 2. 75 − 28 = ?

    • A. 47
    • B. 53
    • C. 57
    • D. 43
    Show answer
    Answer: 4775 − 30 = 45, add 2 = 47.

  3. 3. Counting up can find the difference.

    • A. True
    • B. False
    Show answer
    Answer: TrueThat is a subtraction strategy.

  4. 4. 81 − 19 = 72

    • A. True
    • B. False
    Show answer
    Answer: False81 − 20 = 61, add 1 = 62.

  5. 5. 100 − 40 = ______

    Show answer
    Answer: 6010 tens − 4 tens = 6 tens.

  6. 6. Use counting up to find 60 − 45.

    Show answer
    Answer: 45 + 5 = 50, + 10 = 60; 15Count from 45 to 60.

Homework

  1. When an adult buys something, work out the change in your head.