Cool Multiplication Trick Part 1

We have learned quite a number of multiplication tricks now and in this post, I am going to teach you another multiplication trick which you can use to calculate faster and share with your friends. This multiplication trick is quite similar to squaring a number ending in 5.

The conditions to perform this trick are

(a) the sum of the ones digit is 10.

(b) the other digits (tens, hundreds, and so on) are the same.

Example 1: 23 × 27

This example fits the two conditions above. For (a), 3 + 7 = 10. Then, for (b) the tens digits are the same.

The Math Trick

Step 1:  Add 1 to one of the tens digit, then multiply to the other tens digits. 

In 23 × 27 That is (2 + 1) × 2 = 3 × 2 = 6

Step 2: Multiply the ones digits.

3 × 7 = 21.

Step 3: Append the digits 621.

So, 23 × 27 =621.

Example 2: 62 × 68

Again, this example satisfies the two conditions above.  So,

Step 1: Add 1 to one of the tens digits and then multiply to the other tens digit.

(6 + 1) × 6 = 7 × 6 = 42

Step 2:  Multiply the ones digits.

2 × 6= 16

Step 3: Append the digits: 4216.

So, 62 × 68 = 4216.

Example 3: 81  × 89

This satisfies condition (a) and (b). So,

Step 1: Add 1 to one of the tens digits and then multiply to other tens digits.

(8 + 1) × 8 = 72

Step 2: Multiply the ones digits

× 9 =9

Step 3: Append the digits.

729?

Nope. This is where your estimation skills comes in. As we can see,

80 × 80

is already equal to 6400, so there is no way that 81  × 89 = 729.

Actually, the correct answer is 7209.

In this multiplication trick, in case the ones digits are 1 and 9, you must add 0 before the 0 in the product. Therefore, 51  × 59 = 3009, not 309

Example 4: 206 × 204

This multiplication also works for higher number of digits such as in this example. In this example, we will treat the hundreds and tens digits as one number. So, step 1 will include multiplication of 20 by 21.

Step 1:  (20 + 1) × 20 = 21 × 20 = 420

Step 2: 6 × 4 = 24

Step 3: 42024

So, 206 × 204 = 42024.

With enough practice, you can use this trick to perform mental multiplication. Don’t forget to share it with your friends.

In the next post, we are going to learn why this method works. Again, it is cool to learn the math trick. But it is awesome to know why the math trick works.

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