There are so many calculations to be performed
and whats to be done when we don't have a calculator. BetaTheta
brings to you an assorted collection of useful Tricks that
will surely help you increase your problem solving speed.
Some of the tricks below are a little tough so please pay
extra attention while reading them. We have more tricks
on page 2.
Trick 1 : Multiply any two numbers from 11 to
20 in your head.
Take 15 x 13 for
example..
Place the larger no. first in your mind and then do something
like this Take the larger no on the top and the second digit
of the smaller no. in the bottom.
15
3
The rest is quite simple. Add 15+3 = 18 .
Then multiply 18 x 10 = 180 ...
Now multiply the second digit of both the
no.s (ie; 5 x 3 = 15) Now add 180 + 15
Here is the answer 180 + 15 = 195 .
Think over it. This is a simple trick. It
will help you a lot.
Trick 2 : Multiply any two digit number with 11.
This trick is much simpler than
the previous one and it is more useful too. Let the number
be 27 . Therefore 27 x 11
Divide the number as 2 _ 7
Add 2+ 7 = 9
Thus the answer is 2 9 7
Wasn't this one simple. But there is one complication.
If you take a number like 57 Thus _57 x 11
Divide the number as 5 _ 7
Add 5 + 7 = 12
Now add 1 to 5 and place 2 in the middle so
the answer is 5+1_2 _7 = _627
Thus the answer is 627
Trick 3 : Multiply any number from 1 to 10 by
9
To multiply by 9,try this:
(1) Spread your two hands out and place them on a desk or
table in front of you.
(2) To multiply by 3, fold down the 3rd finger from the
left. To multiply by 4, it would be the 4th finger and so
on.
(3) the answer is 27 ... READ it from the two fingers on
the left of the folded down finger and the 7 fingers on
the right of it.
This one was really cool wasn't it
Trick 4 : Square a two digit number ending in
five
This one is as easy as the previous ones but you have to
pay a little more attention to this one . Read carefully
:
Let the number be 35
35 x 35
Multiply the last digits of both the numbers
; thus ___ 5 x 5 = 25
now add 1 to 3 thus 3 + 1 = 4
multiply 4 x 3 = 12
thus answer 1225
You will have to think over this one carefully.As
5 has to come in the end so the last two digits o the answer
will be 25 . Add 1 to the first digit and multiply it by
the original first digit . Now this answer forms the digits
before the 25. Thus we get an answer .
Trick 5 : Square any two digit number
Suppose the number is 47 . Look for the nearest
multiple of 10 . ie; in this case 50 . We will reach 50
if we add 3 to 47. So multiply (47+3) x (473) = 50 x 44
= 2200 This is the 1st interim answer.
We had added 3 to reach the nearest multiple
of 10 that is 50 thus 3x 3 = 9 This is the second interim
answer.
The final answer is 2200 + 9 = 2209 ... Practice
This one on paper first.
Trick 6 : Multiply any number by
11 .
Trick number 2 tells you how to multiply a
two digit number by 11 but what if you have a number like
12345678 . Well that is very easy if you our trick as given
below . Read it carefully.
Let the number be 12345678 __ thus 12345678
x 11
Write down the number as 012345678 ( Add a
0 in the beginning)
Now starting from the units digit write down
the numbers after adding the number to the right
So the answer will be 135802458
This one is simple if you think over it properly all you
got to do is to add the number on the right . If you are
getting a carry over then add that to the number on the
left. So I will tell you how I got the answer . Read carefully.
The number was 12345678 ___ I put a 0 before the number
____ so the new number 012345678
Now I wrote ___ 012345678
Then for the answer
8 + 0 = 8
7 + 8 = 15 (1 gets carry carried over)
6+1+7 = 14 ( 1 gets carried over)
5 + 1 + 6 = 12 ( 1 gets carried over)
4 + 1 + 5 = 10 ( 1 gets carried over)
3 + 1 + 4 = 8
2 + 3 = 5
1 + 2 = 3
0 + 1 = 1
Thus the answer = 135802458
Now I hope you have understood the trick if
you want more help please contact me by email using the
contact form given below. Please don't fail to mention your
email address as it is essential to contact back.
Trick 7 : Square a 2 Digit Number, for this example
37:
Look for the nearest 10 boundary
In this case up 3 from 37 to 40.
Since you went UP 3 to 40 go DOWN 3 from 37 to 34.
Now mentally multiply 34x40
The way I do it is 34x10=340;
Double it mentally to 680
Double it again mentally to 1360
This 1360 is the FIRST interim answer.
37 is "3" away from the 10 boundary 40.
Square this "3" distance from 10 boundary.
3x3=9 which is the SECOND interim answer.
Add the two interim answers to get the final answer.
Answer: 1360 + 9 = 1369 

With practice this can easily be done in your head.
It is quite possible that you have problems in
understanding some of the tricks mentioned in this page.
Please use our Reference Forums for
help in understanding any of the above tricks or to ask
for help in any other topic. 