Some important number systems are as follows.
Decimal number System
The Decimal Number System consists of ten digits from 0 to 9. These digits can be used to represent any numeric value. The base of decimal number system is 10. It is the most widely used number system. The value represented by individual digit depends on weight and position of the digit.
Let's take a closer look at something most of us take for granted. Counting in decimal numbers. For example, when you see the decimal number:
4275
it means that you sum the following to get a concept of the total quantity of things represented by that number:
Thousands  Hundreds  Tens  Ones 
4 x 1000  plus 2 x 100  plus 7 x 10  plus 5 x 1 
which is the same as:
4 x 10^{3}  plus 2 x 10^{2}  plus 7 x 10^{1}  plus 5 x 10^{0} 
(Remember that in mathematics, any number raised to the zero power equals 1)
The number that is the on the far right is in the "ones" place holder (called the least significant digit). To the left of that is the "tens" place holder. To the left of that is the "hundreds" place holder, and so on. They are all powers of 10. Whatever digit is sitting in a particular place holder, you multiply that place holder amount ( power of 10) by the digit, and then add them all up.
^{Thousands column}  ^{Hundreds column}  ^{Tens column}  ^{Ones column} 
10^{3}  10^{2}  10^{1}  10^{0} 
digit from 09 x 1000  digit from 09 x 100  digit from 09 x 10  digit from 09 x 1 
4  2  7  5 
= 4 thousand, 2 hundred, and seventyfive
= 4275
Binary Number System
Digital computer represents all kinds of data and information in the binary system. Binary Number System consists of two digits 0 and 1. Its base is 2. Each digit or bit in binary number system can be 0 or 1. A combination of binary numbers may be used to represent different quantities like 1001. The positional value of each digit in binary number is twice the place value or face value of the digit of its right side. The weight of each position is a power of 2.
In binary (Base2), all the columns are powers of 2. Think of the following table as the four least significant digit place holders for the binary number system just like the tables above using the decimal number system:
^{Eights column}  ^{Fours column}  ^{Twos column}  ^{Ones column} 
2^{3}  2^{2}  2^{1}  2^{0} 
The far right colum is the "ones" place holder. To the left of that is the "twos" place holder. To the left of that is the "fours" place holder (2^{2} = 4). To the left of that is the "eights" place holder (2^{3} = 8). They are all powers of 2. Whatever digit is sitting in a particular place holder, you multiply that place holder amount ( power of 2) by the digit, and then add them all up. However, remember that a binary digit has only two options, 0 or 1, so it's very easy.
For example, let's look at the binary number:
1011
You would this interpret in decimal as:
^{Eights}  ^{Fours}  ^{Twos}  ^{Ones}  
1 x 2^{3}  plus 0 x 2^{2}  plus 1 x 2^{1}  plus 1 x 2^{0}  
which equals  1 x 8  plus 0 x 4  plus 1 x 2  plus 1 x 1 
which equals  8  plus 0  plus 2  plus 1 
The total is: 11 (in decimal) which equals 8 + 0 + 2 + 1
In other words,
1011_{2} = 11_{10}
(Note: the subscripted number indicates the number base)
Octal Number System
Octal Number System consists of eight digits from 0 to 7. The base of octal system is 8. Each digit position in this system represents a power of 8. Any digit in this system is always less than 8. Octal number system is used as a shorthand representation of long binary numbers. The number 6418 is not valid in this number system as 8 is not a valid digit.
Let's dig right in:
I will do the usual sequential division, this time dividing by 8 at each step:
Once I got to the "5" on top, I had to stop, because 8 doesn't divide into 5.
Then the corresponding octal number is 545_{8}.
I will follow the usual procedure, listing the digits in one row and then counting off the digits from the RIGHT in the next row, starting at zero:
digits:  5  4  5 
numbering:  2  1  0 
Then I'll do the usual addition and multiplication:
5×8^{2} + 4×8^{1} + 5×8^{0}
= 5×64 + 4×8 + 5×1
= 320 + 32 + 5
= 357
Then the corresponding decimal number is 357_{10}.
Hexadecimal Number System
The Hexadecimal Number System consists of 16 digits from 0 to 9 and A to F. The alphabets A to F represent decimal numbers from 10 to 15. The base of this number system is 16. Each digit position in hexadecimal system represents a power of 16. The number 76416 is valid hexadecimal number. It is different from 76410 which is seven hundred and sixty four. This number system provides shortcut method to represent long binary numbers.
Here, I will divide repeatedly by 16, keeping track of the remainders as I go. (You might want to use some scratch paper for this.)
Reading off the digits, starting from the top and wrapping around the righthand side, I see that:
357_{10} = 165_{16}.
List the digits, and count them off from the RIGHT, starting with zero:
digits:  1  6  5 
numbering:  2  1  0 
Remember that each digit in the hexadecimal number represents how many copies you need of that power of sixteen, and convert the number to decimal:
1×16^{2} + 6×16^{1} + 5×16^{0}
= 1×256 + 6×16 + 5×1
= 256 + 96 + 5
= 357
Then 165_{16} = 357_{10}.
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