Subtraction (110–120) — Subtracting Within 20 and Number Practice – Primary 1 Basic Science Lesson Note

TOPIC: Subtraction (110–120)
SUBTOPIC: Subtracting 1-digit and 2-digit Numbers (Within 20)
CLASS: Primary 1
DATE: ____________________
PERIOD: ____________________
DURATION: 40 minutes

LEARNING OBJECTIVES

By the end of this lesson, pupils should be able to:

1. Identify and count numbers from 110 to 120.
2. Explain the concept of subtraction as “taking away” or “removing.”
3. Identify the minuend, subtrahend, and difference in subtraction problems.
4. Subtract 1-digit numbers from 1-digit and 2-digit numbers (within 20).
5. Count forward and backward in 2s and 3s.
6. Apply subtraction in simple real-life situations.

KEY VOCABULARY WORDS

Subtract, Minus (–), Equals (=), Difference, Take away, Count backward, Less than, Minuend, Subtrahend.

INSTRUCTIONAL MATERIALS

• Number chart (1–120)
• Counters or bottle tops
• Flashcards showing subtraction facts (e.g., 10 – 3 = 7)
• Number line (0–20)
• Classroom objects (erasers, pencils, books)
• Real-life picture examples (fruits, toys, etc.)

INTRODUCTORY ACTIVITY

1. Display flashcards of numbers 110–120 and have pupils count them aloud.
2. Ask pupils to count backward from 120 to 110.
3. Show 10 bottle tops, remove 3, and ask:
   • “How many are left?”
   • Explain that Subtraction means taking away to find what remains.
   • Demonstrate: 10 – 3 = 7

CONTENT OF THE LESSON

1. Counting and Identifying Numbers (110–120)

📊 [Insert colourful number line chart 110–120]

1. Meaning of Subtraction

Subtraction means taking away or removing a number from another number to find what is left.
We use the minus sign (–) for subtraction.
Example:

• 8 – 3 = 5 → “If you take 3 away from 8, 5 is left.”

Parts of subtraction:

• Minuend: The number you start with (the bigger number).
• Subtrahend: The number being taken away.
• Difference: The answer after subtraction.

🧩 Example: 9 – 4 = 5
→ 9 is the minuend, 4 is the subtrahend, and 5 is the difference.

📘 [Insert diagram: 9 apples with 4 crossed out to show 5 left]

1. Subtracting Numbers Within 20

📊 [Insert diagram: number line 0–15 showing subtraction 12 – 4 = 8]

1. Subtraction on a Number Line

To subtract on a number line:

1. Start at the first (bigger) number.
2. Move left the number of steps equal to the number being subtracted.

Example: 10 – 3 = ?

• Start at 10.
• Move 3 steps left: 9, 8, 7.
  ✅ Answer = 7

📈 [Insert number line diagram from 0–10 showing steps left]

1. Counting Forward and Backward in 2s and 3s

In 2s (forward): 2, 4, 6, 8, 10, 12, 14, 16, 18, 20
In 2s (backward): 20, 18, 16, 14, 12, 10, 8, 6, 4, 2

In 3s (forward): 3, 6, 9, 12, 15, 18
In 3s (backward): 18, 15, 12, 9, 6, 3

👣 [Insert diagram: colourful steps showing counting in 2s and 3s]

CLASS EXERCISE 1

1. Count and write numbers from 110 to 120.
2. Fill in the missing numbers:
   a. 110, ___, 112, ___, 114, ___, 116, ___, 118, ___
3. Count backward from 120 to 110.
4. Write the number names of 115, 118, 120.

CLASS EXERCISE 2

1. Subtract the following:
   a. 6 – 2 = ______
   b. 8 – 5 = ______
   c. 10 – 4 = ______
   d. 15 – 7 = ______
   e. 12 – 9 = ______
2. On a number line, solve:
   a. 9 – 3 = ?
   b. 10 – 6 = ?

CLASS EXERCISE 3 (Application)

1. If there are 10 mangoes and you eat 4, how many are left?
2. There were 12 pencils in a box. 5 were taken out. How many pencils remain?

ASSIGNMENT (MULTIPLE-CHOICE QUESTIONS)

(Written in one straight line as requested)

1. What is 9 – 4? (a) 3 (b) 4 (c) 5 (d) 6
2. The number before 115 is ______ (a) 114 (b) 116 (c) 113 (d) 112
3. 8 – 3 = ______ (a) 4 (b) 5 (c) 6 (d) 3
4. In 10 – 6 = 4, the number 6 is called ______ (a) Difference (b) Subtrahend (c) Minuend (d) Sum
5. Counting backward means ______ (a) Moving forward (b) Going up (c) Going down (d) Moving left

BOARD SUMMARY

• Subtraction means taking away.
• The minus sign (–) shows subtraction.
• 9 – 4 = 5 means when 4 is taken from 9, 5 is left.
• Minuend = the number you start with.
• Subtrahend = the number being taken away.
• Difference = the answer.