Lesson Notes By Weeks and Term v3 - Primary 1
Subtraction I
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Subtraction I
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Subject: General Mathematics
Class: Primary 1
Term: 1st Term
Week: 6
Theme: Basic Operations
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Watch on YouTubesubtract from whole numbers not greater than 9; subtract from whole numbers not greater than 18; crosscheck accuracy in subtraction.
Greater Than 18 This extends the same methods to slightly larger numbers, requiring careful counting. Worked Example 1 (Using Tally Marks/Drawings): Problem: There are 12 eggs in a basket. A cook uses 4 eggs. How many eggs are left?
Step 1: Draw 12 tally marks or circles: |||| |||| || (or OOOOOOOOOOOO)
Step 2: Cross out 4 tally marks/circles: XXXX |||| || (or XXXXXXXXOOOO)
Step 3: Count the remaining tally marks/circles: 8 Answer: 8 eggs are left. (12 – 4 = 8). Worked Example 2 (Using Counting Backwards or combination of fingers and objects): Problem: 15 – 7 = ?
Method 1 (Counting Backwards): Start at
1
5. Count back 7 steps: 14, 13, 12, 11, 10, 9,
8. Method 2 (Visualizing in groups of ten and ones or breaking down): Imagine 10 fingers and 5 more. Take away 5 first from the 10 and then 2 from the 5 remaining or vice-versa. Start with 15 objects. Remove 7 objects. Count the remaining
8. Answer: 15 – 7 =
8. F. Cross-checking Accuracy in Subtraction This involves verifying if the answer to a subtraction problem is correct. The simplest method for Primary 1 is using addition.
Principle: If 'a – b = c', then 'c + b' must equal 'a'. (The difference plus the number taken away should give the starting number).
Worked Example 1: Problem: Musa calculated 9 – 3 =
6. How can he check his answer?
Step 1: Take the answer (6) and the number that was subtracted (3).
Step 2: Add them together: 6 +
3. Step 3: The sum is
9. This matches the original starting number (9).
Conclusion: The answer is correct.
Worked Example 2: Problem: Check if 14 – 6 = 7 is correct.
Step 1: Take the proposed answer (7) and the number subtracted (6).
Step 2: Add them together: 7 +
6. Step 3: The sum is
1
3. Step 4: This (13) does not match the original starting number (14). * Conclusion:** The answer 14 – 6 = 7 is incorrect. (The correct answer is 8, because 8 + 6 = 14). A. What is Subtraction? Subtraction is the process of taking one number or quantity away from another. It tells us how many are left, the difference between two numbers, or how much less one number is than another.
Keywords: 'Take away', 'minus', 'difference', 'fewer', 'how many left', 'subtract'.
Symbol: The minus sign (–) is used to represent subtraction. For example, 5 – 2 means "5 take away 2". B. Components of a Subtraction Problem (Simplified for Primary 1) While formal terms like 'minuend', 'subtrahend', and 'difference' might be too advanced, teachers should ensure students understand: The starting number: The total amount one begins with (e.g., "You have 7 mangoes").
The number to be taken away: The quantity that is removed (e.g., "You eat 2 mangoes").
The result/remaining number: What is left after taking away (e.g., "How many mangoes are left?").
C. Methods for Subtraction (Practical for Nigerian Classrooms)
1. Using Concrete Objects: This is the most effective method for Primary
1. Procedure: Teacher presents a group of objects (e.g., counting sticks, bottle tops, pebbles, seeds, fruits). Students count the total. Teacher then physically removes a specified number of objects. Students count the remaining objects.
Example: Teacher places 6 stones on the table. "We have 6 stones. Take away 2 stones." Students remove 2 stones. "How many stones are left?" Students count the remaining 4 stones. So, 6 – 2 = 4.
2. Using Fingers: Useful for smaller numbers.
Procedure: Students raise the number of fingers representing the starting number. They then fold down the number of fingers to be subtracted. They count the remaining raised fingers.
Example: For 9 – 4: Students raise 9 fingers. They fold down 4 fingers. They count the 5 remaining raised fingers. So, 9 – 4 = 5.
3. Drawing Pictures/Tally Marks: A visual representation method.
Procedure: Students draw circles, squares, or tally marks to represent the starting number. They then cross out the number of items to be subtracted. They count the uncrossed items.
Example: For 7 – 3: Students draw 7 circles: O O O O O O
O. They cross out 3 circles: X X X O O O
O. They count the 4 remaining circles. So, 7 – 3 = 4.
4. Counting Backwards: Mentally or using a number line (if available/introduced).
Procedure: Starting from the larger number, students count backwards the number of steps to be subtracted.
Example: For 8 – 3: Students start at 8 and count backwards 3 steps: 7, 6,
5. The answer is
5. So, 8 – 3 =
5. D. Subtraction from Whole Numbers Not Greater Than 9 This involves applying the methods above to problems where the initial number is 9 or less.
Worked Example 1 (Using Objects/Fingers): Problem: Femi has 5 biscuits. He eats 2 biscuits. How many biscuits does Femi have left?
Step 1: Start with 5 biscuits. (Show 5 objects or raise 5 fingers).
Step 2: Take away 2 biscuits. (Remove 2 objects or fold down 2 fingers).
Step 3: Count the remaining biscuits. (Count the 3 remaining objects/fingers).
Answer: Femi has 3 biscuits left. (5 – 2 = 3).
Worked Example 2 (Using Drawings): Problem: Subtract 3 from
8. Step 1: Draw 8 circles: O O O O O O O O Step 2: Cross out 3 circles: X X X O O O O O Step 3: Count the remaining circles: 5 Answer: 8 – 3 =
5. E. Subtraction from Whole Numbers Not Greater Than 18 This extends the same methods to slightly larger numbers, requiring careful counting. Worked Example 1 (Using Tally Marks/Drawings): Problem: There are 12 eggs in a basket. A cook uses 4 eggs. How many eggs are left?
Step 1: Draw 12 tally marks or circles: |||| |||| || (or OOOOOOOOOOOO)
Step 2: Cross out 4 tally marks/circles: XXXX |||| || (or XXXXXXXXOOOO)
Step 3: Count the remaining tally marks/circles: 8 Answer: 8 eggs are left. (12 – 4 = 8). *Worked Example 2
A. Teacher Activities: Introduction (Engage): Begin by asking students about situations where they 'take away' things (e.g., "I had 5 pencils, I gave 1 to my friend. How many are left?").
Use local context: "If Mummy bought 7 oranges and we ate 3, how many remain?" Concept Demonstration (Explore): Present a collection of concrete objects (e.g., 8 bottle tops, 5 chalks, 10 seeds). Demonstrate subtraction using these objects: "We have 8 bottle tops. I take away
3. How many are left?" (Physically remove 3 and count).
Introduce the minus sign: "8 minus 3 equals 5." Write on the board: 8 – 3 =
5. Repeat with different numbers within
9. Demonstrate using fingers for smaller numbers (e.g., 7 – 2). Demonstrate drawing pictures/tally marks for numbers up to 18 (e.g., 12 – 5).
Guided Practice (Explain): Write subtraction problems on the board (e.g., 6 – 4 = ?, 9 – 5 = ?, 13 – 6 = ?). Guide students to solve them using objects, fingers, or drawings as a class. Circulate, observe, and provide immediate feedback.
Introducing Cross-checking (Elaborate): After solving a problem, e.g., 7 – 3 = 4, ask: "How can we be sure our answer is correct?" Demonstrate checking by adding the answer to the number taken away: "Let's add 4 and
3. What do we get? 7! Is 7 our starting number? Yes! So our answer is correct." Repeat with another example, including one where the initial answer is wrong to show why checking is important.
B. Student Activities: Object Manipulation: Students will work individually or in small groups with concrete objects (pebbles, counting sticks, bottle tops, seeds) to physically perform subtraction for given problems.
Finger Counting: Students will use their fingers to perform subtraction for numbers up to 9, and sometimes up to 10 (e.g., 10 – 3).
Drawing/Tally Marks: Students will draw simple pictures or tally marks in their notebooks or on individual slates/whiteboards to solve subtraction problems, especially for numbers greater than 9 up to
1
8. Verbal Participation: Students will respond orally to teacher questions, counting out loud, and stating answers.
Problem Solving: Students will solve guided and independent practice problems using the demonstrated methods.
Cross-checking: Students will practice checking their subtraction answers using addition.
Group Work: In small groups, students can solve word problems related to subtraction, sharing objects and discussing their methods. (Teacher to guide students through these examples, emphasizing the steps and encouraging the use of concrete materials or drawings.)
Problem: Musa had 7 oranges. He ate 3 oranges. How many oranges are left?
Thinking Process: Start with 7 objects (oranges). Take away 3 objects. Count the remaining objects.
Solution: 7 oranges (draw 7 circles: O O O O O O O)
Eat 3 oranges (cross out 3 circles: X X X O O O O)
Count remaining: 4 So, 7 – 3 =
4. Commentary: This helps students connect the concrete action of taking away to the mathematical operation.
Problem: Subtract 5 from
9. Thinking Process: Raise 9 fingers. Fold down 5 fingers. Count remaining fingers.
Solution: Start with 9. (Raise 9 fingers). Take away 5. (Fold down 5 fingers).
Remaining: 4 fingers. So, 9 – 5 =
4. Commentary: Reinforces the use of fingers as a direct manipulative for subtraction.
Problem: Aisha had 12 groundnuts. She gave 4 groundnuts to her friend. How many groundnuts does Aisha have now?
Thinking Process: Draw 12 items (groundnuts). Cross out 4 items. Count the items that are not crossed out.
Solution: Draw 12 tally marks: |||| |||| || Cross out 4 marks: XXXX |||| || Count remaining: 8 So, 12 – 4 =
8. Commentary: Introduces subtraction for numbers greater than 9, using drawings as a primary method.
Problem: Calculate 15 –
7. Thinking Process: Visualize 15 items. Remove 7 items. Count what's left. (Can use drawing 15 circles and crossing out 7, or counting backwards from 15).
Solution: Using drawings: OOOOOOOOOOOOOOO Cross out 7: XXXXXXXOOOOOOOO Count remaining: 8 So, 15 – 7 =
8. Commentary: Further practice with larger numbers, reinforcing methods previously taught.
Problem: If you calculate 10 – 6 = 4, how can you check if your answer is correct?
Thinking Process: Identify the difference (answer) and the number subtracted. Add them together. Compare the sum to the original starting number.
Solution: The answer is
4. The number subtracted is
6. Add them: 4 + 6 =
1
0. The sum (10) is the same as the starting number (10).
Therefore, the answer 10 – 6 = 4 is correct.
Commentary: Explicitly teaches the cross-checking method using addition, a critical skill for accuracy.
Market Scenario (Economic Literacy): When buying food items like groundnuts or fruits, knowing how to subtract helps determine the correct change. For example, if a child buys akara for ₦50 with a ₦100 note, they need to subtract ₦50 from ₦100 to know they should get ₦50 change. If they get less, they can identify the error.
Resource Management (Community/Household): In a household, a mother might need to know how many pieces of yam are left after some have been cooked for dinner. If she started with 15 pieces and cooked 8, she uses subtraction (15 – 8 = 7) to know there are 7 pieces remaining. This helps in planning future meals. Time Management/Counting Days (Cultural/Daily Life): Counting down days to a special event like a holiday (e.g., Christmas, Eid, a local festival) or a child's birthday. If there are 10 days left and 3 days pass, subtraction (10 – 3 = 7) helps to determine 7 days are now remaining.