Multiplication Tricks Part Two

In the previous article of Multiplication Tricks it was shown how to square a number which ends in five. For example square of 15, 25, 35 and so on. In this article I will point out some more multiplication tricks which can be done mentally. Wouldn't you feel good if you have an answer before somebody picks up the calculator, punches the number and comes up with the same answer you have. Here are some tricks

Multiply by 11

Whenever you multiply by 11, add up the digits and put that sum of the digits in between the digits. This is best explained by example.

Example 1: Multiply 25 by 11

Steps to multiply

a. The first digit which is 2

b. sum the first and second digit which is 2+5=7

Now put the sum which is 7 in between the digits which are 2 and 5. So, the answer is 275.

Example 2: Multiply 67 by 11

Steps to multiply

a. The first digit which is 6

b. sum the first and second digit which is 6+7=13

Whenever the sum is more than 9, take the carry over 1 and add it the left digit.

In our example above sum is 13 which is more than 9. So take carryover 1 and add it to left digit which is 6 to get 7. So, the answer is 737

Lets try one more example for two digit multiplication before we move on to three digit multiplication

Example 3: Multiply 99 by 11

Steps to multiply

a. write down the first digit which is 9

b. sum the first and second digit which is 9+9=18

Now the sum is more than 9 (as in above example). So, we take carry over 1 and add it the left digit which is 9 to give us 10.

So, the answer is 1089

Multiplying 3 digit numbers by 11

This is nothing but the extension of above problems â€“ see how it works.

Example 4:Multiply 345 by 11

Steps to multiply

a. write down the first digit 3

b. sum the first and second digit 3+4=7

c. sum the second and third digit 4+5=9 note:

d. the last digit is 5

So, the answer is 3795

.

Example 5: Multiply 567 by 11

Steps to multiply

a. write down the first digit 5

b. sum the first and second digit 5+6=11

c. sum the second and third digit 6+7=13 note:

d. the last digit is 7

This example has carryovers as we discussed above. Lets see how they work

Now from 13 we take carry over 1 and add it to 11 to get 12

Now from 12 we take carry over 1 and add it to 5 to get 6

The answer is 6237

Shown in table format - for easy understanding.

 First digit Sum of 1st and 2nd Sum of 2nd and 3rd Last digit After steps a,b,c and d we have 5 11 13 7 Since 13 is more than 9, so we add carry over 1 to left digit 11and now we have 5 12 3 7 Since 12 is more than 9, so we add carry over 1 to left digit 5. Now we have 6 2 3 7

Example 5: Multiply 998 by 11

Steps to multiply

a. write down the first digit 9

b. sum the first and second digit 9+9=18

c. sum the second and third digit 9+8=17 note:

d. the last digit is 8

Now from 17 we take carry over 1 and add it to 18 to get 19

Now from 19 we take carry over 1 and add it to 9 to get 10

The answer is 10978

Shown in table format - for easy understanding

 First digit Sum of 1st and 2nd Sum of 2nd and 3rd Last digit After steps a,b,c and d 9 18 17 8 Since 17 is more than 9 so we add carry over 1 to left digit 18 and now we have 9 19 7 8 Since 19 is more than 9 so we add carry over 1 to left digit 5. Now we have 10 9 7 8