Multiply by 9999 trick

Here is a nice trick to multiply a 4 digit number by 9999.

For instance, lets try multiplying 1729 by 9999.

First, we reduce 1729 by 1.

1729-1=1728

Then, we subtract each of the above digits from 9 to get 8271.

(9-1=8, 9-7=2, 9-2=7, 9-8=1)

In conclusion, 9999 x 1729=1728,8271.

Impressive isn’t it?

This trick works for many 9s too, for example multiplying by 99999999.

To multiply 9999 with a number with less digits, for instance, 12, simply pad zeroes in front of the number, to become 0012.

Then, using the above method, 9999 x 0012 = 0011,9988=119988.

Secrets of Mental Math: The Mathemagician’s Guide to Lightning Calculation and Amazing Math Tricks