Finding Missing Numbers in Open Sentences for Primary 1

Finding Missing Numbers in Open Sentences for Primary 1

Performance Objectives

By the end of the lesson, students should be able to:

1. find missing numbers in a statement;
2. cross-check the correctness of addition in everyday life.

What is an Open Sentence?

An open sentence in mathematics is a number statement that has a missing number. This missing number is usually shown by a blank space or an empty box.

Finding the missing number is like solving a small puzzle. The goal is to find the number that makes the statement true.

For example, look at this statement:

$$ 5 + \text{ } = 7 $$

This is an open sentence because there is a missing number. We need to find the number that, when added to 5, gives us 7.

Finding Missing Numbers in Addition Sentences

To find a missing number in an addition sentence, we can use a simple method called “counting on”. This method helps us find how many more we need to reach the total.

Method: Counting On

The “counting on” method involves these steps:

1. Start counting from the number you already have.
2. Count upwards, one by one, until you reach the total number given in the sentence.
3. The number of counts you made is the missing number.

We can use objects like pebbles, bottle tops, or fingers to help us count.

Worked Examples

Example 1: Find the missing number in $6 + \text{ } = 9$.

The procedure involves the following steps:

1. Start with the number 6.
2. Count on from 6 until you reach 9.
3. Count: 7 (that’s 1 count), 8 (that’s 2 counts), 9 (that’s 3 counts).
4. You counted 3 times. So, the missing number is 3.

This means $6 + 3 = 9$.

Example 2: Find the missing number in $4 + \text{ } = 9$.

The procedure involves the following steps:

1. Start with the number 4.
2. Count on from 4 until you reach 9.
3. Count: 5 (1 count), 6 (2 counts), 7 (3 counts), 8 (4 counts), 9 (5 counts).
4. You counted 5 times. So, the missing number is 5.

This means $4 + 5 = 9$.

Example 3: Find the missing number in $3 + \text{ } = 8$.

The procedure involves the following steps:

1. Start with the number 3.
2. Count on from 3 until you reach 8.
3. Count: 4 (1 count), 5 (2 counts), 6 (3 counts), 7 (4 counts), 8 (5 counts).
4. You counted 5 times. So, the missing number is 5.

This means $3 + 5 = 8$.

Example 4: Find the missing number in $2 + \text{ } = 7$.

The procedure involves the following steps:

1. Start with the number 2.
2. Count on from 2 until you reach 7.
3. Count: 3 (1 count), 4 (2 counts), 5 (3 counts), 6 (4 counts), 7 (5 counts).
4. You counted 5 times. So, the missing number is 5.

This means $2 + 5 = 7$.

Cross-Checking Addition in Everyday Life

Cross-checking means checking your answer to make sure it is correct. It is very important to check your addition in many daily activities to avoid mistakes.

Why Cross-Check?

Checking your addition helps you to be sure that the numbers add up correctly. This is important when you are dealing with money, counting items, or keeping scores.

Areas Where Accuracy of Addition is Required

Here are some areas in daily life where it is important to cross-check addition:

1. Counting Money: When you buy things at the market or in a shop, you need to add the prices of the items to know the total cost. If you add incorrectly, you might pay too much or too little. For example, if you buy a biscuit for N5 and a sweet for N3, you add $5 + 3 = 8$. You should cross-check to make sure the total is N8.

2. Counting Items: When you are counting toys, fruits, or other objects, you need to add them correctly to know the total number. For example, if you have 4 red balls and 3 blue balls, you add $4 + 3 = 7$. You should cross-check to make sure you have 7 balls in total.

3. Keeping Scores in Games: When playing a game, you often need to add points to keep track of scores. If you add the scores incorrectly, the wrong person might win the game. For example, if you score 5 points in the first round and 4 points in the second round, you add $5 + 4 = 9$. You should cross-check to ensure your total score is 9.