Using Mental Math for Multiplication

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  • 0:00 Mental Calculation
  • 0:46 Multiplying Single…
  • 2:27 Multiplying Single…
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Do you want to impress your friends and family with your amazing brain? You can when you learn a really easy way to multiply single digits by multiples of 10 and 100 in this lesson.

Mental Calculation

You know, multiplication can be pretty scary. If I asked you to multiply 83,729 by 193,788 that would be hard, even on paper. But what if I asked you to multiply stuff in your head? You can't use a calculator. You can't use a pencil or paper. You just have to think about what the answer would be in your head. Man, that would be difficult, wouldn't it?

And while that sounds scary, I promise that soon enough you will become a mental multiplication wizard as you learn how to multiply single digits by tens and hundreds. You'll impress all of your friends and family with your incredible brain!

Multiplying Single Digits by Tens

Okay, so let's just say that the single digit we want to start working with is lucky 7! And let's also say that you want to multiply lucky 7 by a multiple of 10 less than 100. A multiple of a number is a number that can be exactly divided by that number. The number 40 is a multiple of 10 because 40 can be divided exactly by 10 with no remainder, so let's use 40 for our math problem.

7 x 40 = ?

What do you do first in your head?

  1. Don't panic.
  2. Seriously, don't panic.
  3. Forget that the zero after the 4 in 40 even exists. Pretend the multiplication problem is simply one of single digits. What's 7 x 4? It equals 28!
  4. Remember how we conveniently forgot that the 0 in 40 even exists in step 3? Time to remember it! All you need to do is simply attach the forgotten 0 to the back of 28 to get 280.

The answer to 7 x 40 is 280. And in reality, it only took you two steps to figure that out in your head - if you didn't panic!

Let's try another example to solidify what you just learned. Let's say you want to multiply 3 by 70. This is like multiplying 3 by 7 to get 21. Now, you simply attach a 0 to the back of 21 to get 210. 210 is your answer.

Easy, huh?

Multiplying Single Digits by Hundreds

Okay, now what if I were to try and scare you and ask you to multiply 3 by 700? That's harder right? I mean, we're multiplying by hundreds now. Hundreds!

Let's figure out how to multiply this completely in our heads.

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