Multiplication Shotcuts 2

Multiplication of two digit numbers


Multiplying two digit numbers using column method or by using calculator shall not work for competitive exams. For the former, you will not have time and for the latter, you will not have a calculator in hand :p. So let us see some shortcuts to multiply two digit numbers.

After practicing these methods you will never need a pen to do the math. Victoryyy!!!!

1. By splitting up of number according to our convenience
  

    Method 1 : Multiplication using multiples.

    Example 1 : 17 * 18
  
    Here you can split 18 as 6 * 3. Now our question becomes 17 * 6 * 3

    Multiplication with a single digit is far easier. 
  
    17 * 6 = 102
    102 * 3 = 306

   Method 2 : Multiplication using distribution 

    Example 2 : 17 * 15

    In this method we will split 15 as 10 + 5 . Multiplying some number with 10 ad 5 is                        pretty easy compared to ther numbers.

   17 * 15 = 17 * 10 + 17 * 5
     
   17 * 10 = 170
   17 * 5 = 85
   
   170 + 85 = 255.

2. Multiplication by 15 (For even numbers).

   We saw in the previous example that when multiplying with 15 what you do is you are basically           multiplying the given number with 10 and 5 and then adding them both, There is an easier way to
   do the same.

   1. Take half of the number.
   2. Add it to the number itself.
   3. Multiply with 10 ( or perhaps, put a zero at the end ).

   Example 3: 48 * 15
   
   1. 48/2 = 24.
   2. 48 + 24 = 72.
   3. 72 * 10 = 720.

   i.e 48 * 15 = 720.



    

Multiplication shortcuts - 1

Multiplication of Two digit number with a Single digit number

We've been doing this since the childhood. 32*8

But when it comes to competitive exams there isn't time to do the colum method. You needed step marks when a kid. Not anymore :p

So lets see how to do the multiplication in our mind itself.

Any two digit number 'ab' can be split as (a*10+b). b is the unit's place digit and a is the ten's place digit. When multiplying with a single digit number we split the two digit number like this.

for eg. 32 * 8 =  ( 3* 10 + 2 ) * 8

Now our problem is simplified.

1. First multiply 3 with 8. 8 * 3 = 24.

2. Now put a zero at the end. 240.

3. Multiply 2 with 8. 2 * 8 =16.

4. Add the number obtained now with the previously obtained number.

240 + 16 = 256.

Seeing the four steps one may think, "how hectic is this!"  Try out two questions on your own and you shall find how easy this method is :D

Lets see another example..

Example : -    49*8

49 = ( 4 * 10 )+ 9

1. 4*8 = 32.

2. 320.

3. 9 * 8 = 72

4. 320 + 72 = 392.