Mastering the Trick to Multiply by 11

In this video, PR Haller demonstrates a math trick for multiplying any number by 11 in your head. The trick involves copying the first digit, adding the digits, and copying the last digit. For numbers with carry over, adjustments are made. The method is applicable to larger numbers as well. A brief proof explains the logic behind the trick. The video concludes with a call to subscribe and explore more resources.

How do you multiply a single-digit number by eleven?

• Write the number three times.

• Subtract the number from eleven.

• Write the number two times.

• Add the number to itself.

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CCSS.3.NBT.A.3

What is the first step in multiplying a two-digit number by eleven?

• Copy the first digit of the number.

• Multiply the number by ten.

• Copy the last digit of the number.

• Add the two digits together.

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CCSS.4.NBT.B.5

What is the result of multiplying 17 by eleven?

• 207

• 197

• 177

• 187

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CCSS.4.NBT.B.5

What is the result of 26 times eleven?

• 266

• 276

• 256

• 286

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CCSS.4.NBT.B.5

What do you do if adding the digits of a two-digit number results in a number greater than 9?

• Restart the calculation.

• Carry over to the next digit.

• Subtract ten from the result.

• Ignore the carry.

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How do you handle carrying over when the sum of two digits is more than 9?

• Add it to the next digit.

• Ignore it.

• Multiply it by the next digit.

• Subtract it from the next digit.

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CCSS.1.NBT.C.4

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What is the result of 49 times eleven?

• 549

• 529

• 539

• 559

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CCSS.4.NBT.B.5

What is the result of multiplying 123 by eleven?

• 1343

• 1363

• 1333

• 1353

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CCSS.4.NBT.B.5

How do you multiply a three-digit number by eleven?

• Copy the first and last digits, then add the in-between numbers.

• Multiply each digit by eleven individually.

• Add all digits and place the sum in the middle.

• Divide the number by eleven first.

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CCSS.4.NBT.B.5

Why does the trick for multiplying by eleven work?

• It's based on the distributive property of multiplication over addition.

• Due to the commutative property of multiplication.

• Because eleven is a prime number.

• Because of the properties of addition.

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CCSS.3.OA.B.5