Quantitative Aptitude

I would teach you how you can multiply mentally for numbers from 11 to 19 without using any paper.

So lets start with a sample first.

Lets multiply 17x19

First you have to put the larger number front. i.e. 19x17 is what we would do.
First Add 19+7 i.e. we get 26.
Now multiply it with 10 i.e. it becomes 260.
Now multiply the units digit of both the numbers. i.e. 9x7 = 63.
Add both and thats your answer i.e. 260+63 = 323.

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To quickly multiply by 5, divide the number in two and then multiply it by 10. This can be done quickly in your head.

How it works.

We can write 5 as 1/2x10

So now lets say we would like to multiply 28 and 5 and so using the above formula we can

First half 28 i.e. 14
Now multiplying it to 10 gives 140 and so answer is 140.

The same thing can be applied to 50 which is half of 100 i.e. 1/2x100

Generalizing the above...

One of my friend in an interview faced a question related to prime number and so I thought this would help lot of people to do some of the prime number maths quickly.

Divisibility By 2

All the even numbers are divisible by 2. E.g. Numbers ending in 0,2,4,6,8

Divisibility by 3

Add all the digits of the number and see if its divisible by 3. If the sum is divisible by 3, so is the number

Divisibility by 4

If last...