Some Quick Tricks For Mental Maths!

In this post, we will discuss some maths multiplication tricks that students (and adults!) can use to do math in their head!

Multiplication By 5

Instead of multiplying a number you can just divide it by 2 and then, multiply it with 10. It will give you the result much faster! 

Some examples - 

98*5 => (98/2)*10 => 49*10 => 490

44*5 => (44/2)*10 => 22*10 => 220

585*5 => (585/2)*10 => 292.5*10 => 2925

1024*5 => (1024/2)*10 => 512*10 => 5120 

I don't think I need to tell you how this works!

Multiplication by 11

The trick to multiplying a 2 digit number by 11 is that write down the original number with a space between them and, then fill up space with their sum!

For example, 

25*11 => 2 _ 5 => 2/5+2/5 => 275

45*11 => 4 _ 5  => 4/4+5/5 => 495

36*11 => 3 _ 6 => 3/3+6/6 => 396

77*11 => 7 _ 7 => 7/7+7/7 => 7/14/7. As you can see, we can't have the two digits in the middle, we'll just carry forward the 1 and add it to 7. 7+1/4/7 => 847 

Multiplying 3 digits number, and bigger by 11

Just write down the first and the last digit as it is, and leave some space between them, and then from the  last start adding the numbers to their former digit, 

For example,

123*11 => 1 _ _ 3 => 1/1+2/2+3/3 => 1353 

1213*11 => 1 _ _ _ 3 => 1/1+2/2+1/1+3/3 => 13343

110122*11 => 1_____2 => 1/1+1/1+0/0+1/1+2/2+2/2 => 1211342

How this method works?

Let's suppose we have a 2 digit number, xy, that can be written as 10x+y, now,

   x y

 *1 1

 ------

   x y

x y  -

--------

x/x+y/x

--------

and that's how this method works!

Division By 5

While multiplying a number by 10 and divided it by 2, while dividing with 5, we will do the opposite, we will multiply the number by 2 and divide it by 10.

For example,

44/5 => (44*2)/10 => 88/10 => 8.8

98/5 => (98*2)/10 => 196/10 => 19.6

123/5 =>  (123*2)/10 => 246/10 => 24.6

1156/5 => (1156*2)/10 => 2312/10 => 231.2

Division By 25

This time, instead of multiplying by 2, we'll multiply the number by 4 and divide it by 25.

33/25 => 33*4/100 => 132/100 => 1.32

88/25 => 88*4/100 => 352/100 => 3.52
99/25 => 99*4/100 => 396/100 => 3.96