What is a Friendly Number Strategy? | Simple Explanation for Class 5-10

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What is a Friendly Number (Strategy)?

Grade Level:

Pre-School – Class 2

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Definition

What is it?

A Friendly Number (Strategy) is a mental math trick where you change one or both numbers in a calculation to make them easier to work with, like making them end in 0. After calculating with the friendly numbers, you adjust the answer to get the correct result. This strategy helps you solve problems faster in your head.

Simple Example

Quick Example

Imagine you need to add 19 + 7. It's tricky in your head. Using friendly numbers, you can change 19 to 20 (a friendly number!). Now, 20 + 7 is easy: 27. Since you added 1 extra to 19, you must subtract 1 from your answer: 27 - 1 = 26. So, 19 + 7 = 26.

Worked Example

Step-by-Step

Let's calculate 48 + 35 using the Friendly Number Strategy.

1. **Identify the numbers:** We have 48 and 35.
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2. **Make one number friendly:** 48 is close to 50. Let's change 48 to 50. To do this, we added 2 (50 - 48 = 2).
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3. **Perform the easy calculation:** Now add 50 + 35. This is 85.
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4. **Adjust the answer:** Since we added 2 extra in Step 2, we need to subtract 2 from our temporary answer. So, 85 - 2 = 83.
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5. **Final Answer:** Therefore, 48 + 35 = 83.

Why It Matters

This strategy makes mental calculations much faster, which is super useful in daily life. It's a core skill for anyone in finance managing money, engineers quickly estimating materials, or even shopkeepers calculating change. Mastering it helps you build strong number sense and problem-solving skills.

Common Mistakes

MISTAKE: Forgetting to adjust the answer after using friendly numbers. E.g., for 19 + 7, changing 19 to 20, getting 27, and stopping there. | CORRECTION: Always remember to add or subtract the amount you used to make the number 'friendly' from your final answer.

MISTAKE: Changing both numbers to friendly numbers without proper adjustment strategy. E.g., for 48 + 35, changing 48 to 50 (+2) and 35 to 40 (+5), then adding 50 + 40 = 90, and not knowing how to adjust. | CORRECTION: Usually, it's easier to make just one number friendly first. If you make both friendly, keep track of the total change made and adjust accordingly (e.g., if you added 2 and added 5, you added 7 extra in total, so subtract 7 from the sum).

MISTAKE: Choosing unfriendly 'friendly' numbers. E.g., for 27 + 14, changing 27 to 30 (+3) is good. Changing 27 to 40 (+13) is not as helpful. | CORRECTION: Pick friendly numbers that are closest to the original numbers, usually ending in 0 or 5, to keep the adjustment small and easy.

Practice Questions

Try It Yourself

QUESTION: Use the Friendly Number Strategy to calculate 29 + 15. | ANSWER: 29 + 15. Change 29 to 30 (+1). 30 + 15 = 45. Adjust: 45 - 1 = 44. So, 29 + 15 = 44.

QUESTION: A chaiwala sold 57 cups of tea in the morning and 38 cups in the evening. How many cups did he sell in total using the Friendly Number Strategy? | ANSWER: 57 + 38. Change 57 to 60 (+3). 60 + 38 = 98. Adjust: 98 - 3 = 95. So, he sold 95 cups.

QUESTION: Calculate 103 - 18 using the Friendly Number Strategy. (Hint: Think about what happens when you make the number being subtracted friendly). | ANSWER: 103 - 18. Change 18 to 20 (+2). Now, 103 - 20 = 83. Since we subtracted 2 more than we should have (we subtracted 20 instead of 18), we need to add 2 back to our answer. So, 83 + 2 = 85. Therefore, 103 - 18 = 85.

MCQ

Quick Quiz

Which of these is the best first step to solve 36 + 27 using the Friendly Number Strategy?

Change 36 to 30 and 27 to 30.

Change 36 to 40.

Add 30 + 20 directly.

Change 27 to 25.

The Correct Answer Is:

Changing 36 to 40 makes it a friendly number (ending in 0) with a small adjustment (+4). This is the easiest and most efficient first step for this strategy. Options A and D involve larger or less 'friendly' changes, and C skips the strategy.

Real World Connection

In the Real World

When you're at a local kirana store and the shopkeeper quickly calculates the total bill for your groceries in their head, they are often using friendly number strategies. Similarly, when a cricket commentator gives a quick estimate of how many runs are needed per over, they might be using this trick to simplify the numbers mentally.

Key Vocabulary

Key Terms

MENTAL MATH: Calculations done in your head without paper or calculator | STRATEGY: A plan or method used to achieve a goal | ADJUSTMENT: Making a small change to something to make it correct or suitable | NUMBER SENSE: An intuitive understanding of numbers and their relationships | ESTIMATE: To roughly calculate or judge the value, number, quantity, or extent of something

What's Next

What to Learn Next

Great job understanding Friendly Numbers! Next, explore 'Compensation Strategy,' which is very similar but often used for subtraction. You'll see how these mental math tricks build on each other to make you a math whiz!