1.Write the number 12345679 on the board (note the missing 8).
2.Ask the student to name any digit from 1 to 9. Suppose it is 3.
3.Tell the class that if they multiply 12345679 by 27, they will get a big surprise.
Children love surprises. You can see the astonishment on their faces when they get the answer 333333333.
Repeat the stunt with a different digit, say 8.
Now the multiplier is 72 and the product is 888888888.
How do you know what multiplier to give?
Dividing 111111111 by 9 yields 12345679,
so multiplying that number by 9 gives 111111111.
If 111111111 is multiplied by any digit A, the product will be AAAAAAAAA.
So, the multiplier is 9A, where A is the chosen digit.
Since 9 x 12345679 = 111111111,
Then A x (9 x 12345679) = AAAAAAAAA or
(9A) x 12345679 = AAAAAAAAA
For example:
Name any digit from 1 to 9. Suppose it is: 3
The secret number: 3 x 9 = 27
27 x 12345679 = 333333333
Another one:
Name any digit from 1 to 9. Suppose it is: 7
The secret number: 7 x 9 = 63
63 x 12345679 = 777777777
More number tricks:
Magic 9 - This is the easiest way to teach or learn multiples of 9
The Number 5 Trick
