What is a Number Family for 15?

S0-SA1-1225

Grade Level:

Pre-School – Class 2

All domains without exception

Definition

What is it?

A Number Family for a specific number, like 15, is a group of related addition and subtraction facts that use the same three numbers. It shows how these three numbers are connected through basic operations. For 15, it means finding pairs of numbers that add up to 15, and then showing the subtraction facts that come from them.

Simple Example

Quick Example

Imagine you have 15 laddoos. If you give 7 laddoos to your friend, you have 8 left. This means 7 + 8 = 15 and 15 - 7 = 8. Also, if your friend gave you 8 laddoos and you already had 7, you'd have 15. This gives us 8 + 7 = 15 and 15 - 8 = 7. All these four facts (7+8=15, 8+7=15, 15-7=8, 15-8=7) form a number family for 15, using the numbers 7, 8, and 15.

Worked Example

Step-by-Step

Let's find one Number Family for 15 using the numbers 6, 9, and 15.

Step 1: Identify the three numbers in the family. Here, they are 6, 9, and 15. The largest number (15) is the sum or the starting point for subtraction.
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Step 2: Write the first addition fact. Add the two smaller numbers to get the largest number. So, 6 + 9 = 15.
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Step 3: Write the second addition fact. Just swap the order of the two smaller numbers. So, 9 + 6 = 15.
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Step 4: Write the first subtraction fact. Start with the largest number and subtract one of the smaller numbers. So, 15 - 6 = 9.
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Step 5: Write the second subtraction fact. Start with the largest number and subtract the other smaller number. So, 15 - 9 = 6.
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Answer: The number family for 15 using 6, 9, and 15 is: 6 + 9 = 15, 9 + 6 = 15, 15 - 6 = 9, and 15 - 9 = 6.

Why It Matters

Understanding number families helps you see the relationship between addition and subtraction, making mental math much faster. This skill is crucial for everyday tasks like calculating change at a shop or managing your pocket money. It's also a basic building block for careers in finance, data analysis, or even engineering, where quick calculations are often needed.

Common Mistakes

MISTAKE: Students only write addition facts and forget the subtraction facts. | CORRECTION: Remember that a complete number family always includes two addition and two subtraction facts (unless the two smaller numbers are the same, like 7+7=14).

MISTAKE: Using numbers that don't actually add up to the target number (15 in this case). For example, saying 5, 8, and 15 form a family. | CORRECTION: Always check if the two smaller numbers truly add up to the largest number. (5 + 8 = 13, not 15, so this is incorrect).

MISTAKE: Mixing up the largest number with the smaller ones in subtraction, e.g., 6 - 15 = 9. | CORRECTION: In subtraction for a number family, you always start with the largest number (the sum) and subtract one of the smaller parts.

Practice Questions

Try It Yourself

QUESTION: Find the number family for 15 using the numbers 5, 10, and 15. | ANSWER: 5 + 10 = 15, 10 + 5 = 15, 15 - 5 = 10, 15 - 10 = 5

QUESTION: Which number is missing from this number family for 15: 15 - 7 = ___, 7 + ___ = 15? | ANSWER: 8

QUESTION: Create a number family for 15 where one of the smaller numbers is 3. What are the four facts? | ANSWER: The other smaller number must be 12 (15 - 3 = 12). So, the family is: 3 + 12 = 15, 12 + 3 = 15, 15 - 3 = 12, 15 - 12 = 3.

MCQ

Quick Quiz

Which set of numbers forms a number family for 15?

2015-04-10T00:00:00.000Z

2015-06-08T00:00:00.000Z

2015-07-08T00:00:00.000Z

2015-05-09T00:00:00.000Z

The Correct Answer Is:

For a number family for 15, the two smaller numbers must add up to 15. In option C, 7 + 8 = 15. In other options, 4+10=14, 6+8=14, and 5+9=14, none of which equal 15.

Real World Connection

In the Real World

When you're shopping at a local kirana store and buy items costing Rs. 7 and Rs. 8, the shopkeeper quickly calculates your total as Rs. 15. If you give him a Rs. 20 note, he knows he needs to return Rs. 5 (20 - 15 = 5). This quick mental math relies on understanding these basic number relationships, which are like simple number families.

Key Vocabulary

Key Terms

NUMBER FAMILY: A set of related addition and subtraction facts using three specific numbers. | ADDITION FACT: A statement showing two numbers added together to get a sum. | SUBTRACTION FACT: A statement showing a number subtracted from another to get a difference. | SUM: The result of adding two or more numbers. | DIFFERENCE: The result of subtracting one number from another.

What's Next

What to Learn Next

Great job understanding number families for 15! Next, you can explore number families for larger numbers, like 20 or 50. This will build on your foundation and prepare you for more complex math operations and problem-solving in higher grades.