Speed Mathematics

This is for GMAT bees who have just started their preperation. This is taken from various books. Vedic Mathematics and Study materials from TIME, a premier institution that provide training for Indian Institute of Management CAT exam.
Shall be adding new techniques in speed mathematics as i go through various topics. So plz do keep up with this thread. If any of u has got more techniques plz do share it here.

To master the quants section, u need to have Knowledge in every area, speed in solving a question and approach in tackeling the test papers.
Speed is very important in GMAT because you are supposed to attend all the questions in GMAT and unattempted wuestions will lead to high penalty in the total score. I would like to demonstrate certain speed methodes of calculation which will be of great use to you in GMAT.
These techniques are drawn from different sources. i just thought of consolidating the useful techniques here.

Before getting into methodes of speed calculations, there are few aspects to be kept in mind. For getting the most out of speed calculation, u should be thorough in the following

Multiplication tables - (1*15) up to (20*15)
Squares up to 25 any higher square can be calculated easily.
Cubes upto 12
Powers of 2 - up to 12
Powers of 3 - upto 6
Reciprocals of numbers - upto 12
Compliments of 100 (i.e. the differance between 100 and the given two-digit number. eg- 25's compliment is 100-25 = 75)

Some ways of simplifying calculations

Multiplication by 5
For multiplication by 5, you should multiply the number to be multiplied by 10 and then divide it by 2.
eg: 6493 * 5 = 64930/2 = 32465

Multiplication by 25
Multiply the number to be multiplied by 100 and divide it by 4
eg: 6493 * 25 = 649300/4 = 162325

Multiplication by 125
Multiply the number by 1000 and divide by 8
eg: 6493 * 125 = 6493000/8 = 811625
( Alternatively, you can treat 125 as 100+25. So multiplication by 125 can be treated as multiplication by 100 and add to this number - 1/4th of itself because 25 is 1/4th of 100.)

Multiplication by 11
the rule is "for each digit add the right hand side and write the result as the corresponding figure in the product."
For the purpose of applying the rule, it will be easier if you assume that there is one "zero" on either side of the given number.
eg: 7469*11= 074690 (apply the above said rule) = 82159

Calculation of Squares
Getting the square of a number ending in 5 is simple. If the last digit of the number is 5, the last two digits of the square will be 25. Whatever is the earlier part of the number multiply it with one more than itself and that will be the first part of the answer. ( the second part of the answer will be 25 only)

35\(^2\)=1225.Here 3*4 = 12(first part) and the second part of the answer is always 25 so the answer is 1225

45\(^2\) = 2025 First part of th answer is 4 * 5=25 followed by 5*5=25

55\(^2\) = 3025

245\(^2\) = 24*25= 60025; First part - 24*25 = 600, last part - 5*5 = 25

Multiplying two numbers both of which are close to the same power of 10

Suppose we want to multiply 97 with 92. The power of 10 to which these two numbers are close is 100. Here 100 is called as the " base ". Write the two numbers with the differance from the base i.e., 100 (including the sign)
as shown below.

97 --> -3 ( 97 is 100-3 )
92 --> -8 ( 92 is 100-8 )

Then take the sum of the two numbers ( including their signs) along EITHER one of the two diagonals ( it will be same the same in both cases ).
Here diagonal sum is 97-8=92-3=89. This will form the first part of the answer.


The second part of the answer is the product ( along with the sign) of the difference from the power of 10 written for the two numbers, Here it is the profuct of -3 and -8 which is 24.

Now the last step is putting these two parts 89 and 24 together one next to the other.
Here the answer is 8924. That is 92*97=8924

Note:- The product of the two deviations should have asa many digits as the number of zeroes in the basae. In the above example the base is 100 having 2 zeroes so the product of -8 and -3 has 2 digits.