What is Multiplying by Nine?

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Grade Level:

Pre-School – Class 2

All domains without exception

Definition

What is it?

Multiplying by Nine means you are adding a number to itself nine times. It's a quick way to find the total when you have nine groups of the same size. For example, if you have 9 boxes, and each box has the same number of items, multiplying by nine helps you find the total items fast.

Simple Example

Quick Example

Imagine you have 9 friends, and each friend eats 2 ladoos. To find the total number of ladoos eaten, you can multiply 2 by 9. This gives you 18 ladoos, much faster than adding 2 nine times (2+2+2+2+2+2+2+2+2).

Worked Example

Step-by-Step

Let's say a chaiwala sells chai for ₹15 a cup. If he sells 9 cups in an hour, how much money does he earn from those 9 cups?

Step 1: Identify the number being multiplied: The price of one cup is ₹15.
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Step 2: Identify how many times it's multiplied: He sells 9 cups.
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Step 3: Set up the multiplication: 15 x 9.
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Step 4: Multiply the ones digit: 9 x 5 = 45. Write down 5 and carry over 4.
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Step 5: Multiply the tens digit: 9 x 1 = 9. Add the carried over 4: 9 + 4 = 13.
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Step 6: Combine the results. The answer is 135.

So, the chaiwala earns ₹135 from selling 9 cups of chai.

Why It Matters

Multiplying by nine helps you solve everyday problems quickly, from calculating scores in a cricket match to figuring out how many snacks you need for 9 friends. It's a fundamental skill used by engineers designing buildings, shopkeepers managing inventory, and even scientists calculating experiment results.

Common Mistakes

MISTAKE: Forgetting to carry over numbers when multiplying larger digits. For example, in 15 x 9, forgetting to carry over the 4 from 9 x 5 = 45. | CORRECTION: Always remember to carry over the tens digit to the next place value and add it after multiplying the next digits.

MISTAKE: Confusing multiplication with addition. Forgetting that 9 x 2 is 2 added 9 times, not 9 + 2. | CORRECTION: Remember multiplication is repeated addition. 9 x 2 means nine groups of two, not just adding nine and two.

MISTAKE: Incorrectly recalling multiplication facts for 9 (the '9 times table'). For example, thinking 9 x 7 is 60 instead of 63. | CORRECTION: Practice your 9 times table regularly! Use tricks like the 'finger trick' or patterns (digits add up to 9) to remember them better.

Practice Questions

Try It Yourself

QUESTION: If each student needs 3 notebooks, and there are 9 students, how many notebooks are needed in total? | ANSWER: 27 notebooks

QUESTION: A bus travels 65 km in one hour. If it travels for 9 hours at the same speed, how much distance does it cover? | ANSWER: 585 km

QUESTION: Your mobile data plan gives you 2 GB of data every day. If you use this plan for 9 days, and then use an extra 5 GB on the 10th day, what is the total data used over these 10 days? | ANSWER: 23 GB (9 days x 2 GB/day = 18 GB; 18 GB + 5 GB = 23 GB)

MCQ

Quick Quiz

What is 9 multiplied by 12?

108

118

129

The Correct Answer Is:

Multiplying 9 by 12 gives you 108. You can think of it as (10 x 9) + (2 x 9) = 90 + 18 = 108. Options B, C, and D are incorrect calculations.

Real World Connection

In the Real World

Imagine you're tracking the runs scored by 9 different batsmen in a T20 cricket match. If each batsman scores an average of 15 runs, you can quickly estimate the total runs from these batsmen by multiplying 15 x 9. This helps sports analysts understand team performance and strategy.

Key Vocabulary

Key Terms

MULTIPLY: To combine equal groups to find a total | PRODUCT: The result of a multiplication problem | TIMES TABLE: A list of the multiples of a number | FACTOR: A number that is multiplied by another number to find a product

What's Next

What to Learn Next

Now that you understand multiplying by nine, you can explore multiplying by other numbers like ten or eleven. These build on the same principles and help you become even faster at mental math and solving more complex problems!