Principles of Mathematics

More Tips On Mental Multiplication

With flexibility in the arrangement of numbers to be multiplied, mental multiplication can be easily done with numbers that seems to be big.

Example:  21 can be splitted into 20 + 1 = (2 x 10) + 1

With this splitting of “big” number, 21 becomes operations using only 2, 10 and 1. All these numbers are easy to handle mentally.

Mentally we need to double the number, append a”0″ behind, and add back the original number to the answer.

Let’s do an example.

Example-A:  34 x 21

Mental computation is
34 x 2 ==> 68 ==> append “0″ ==> 680 ==> 680 + 34 = 714 (answer).

See it can be done easily!

Example-B:   34 x 15

Here, 15 can be splitted into 10 + 5.

However, 34 x 15 ==> 34 x (10 + 5)  involves 34 x 5 which may be hard to handle.

What is a better way?

15 can be modified to 1.5 x 10.  This looks better as 0.5 is “half” which is 1/2, an easier operation.

Thus 34 x 15 ==> 34 x 1.5 x 10   (All these uses simpler number of 2 and 10).

34 x 15 ==> 34 x ( 1 + half) x 10 ==> [ 34 + (34 / 2) ] x 10  =  [34 + 17] x 10 = 510

Here we see that the “half” method works better than the “x5″ method (mentally).

From the above 2 examples, we can see that multiplication of 2 seemingly complex numbers can be done easily by modifying the numbers to simpler ones having 2, 10 and the likes. Coupled with simpler mathematical operations, these multiplication can be done mentally with ease.