0 - 9 plus A - F
The "x" or "y" or abcdefg or any letter after a number is called a variable, and can be used to represent any number.
Memory dump which are in binary numbers would have many numbers of 0s and 1s. working with these numbers would be very difficult. Hence two number system hexadecimal and octal number system is used because these numbers are inter convertible with binary numbers by the concept of bits.
There are 16 symbols.
It represents a Cheque Number. This is a unique number that is used to identify the cheque. Just by having this number we can identify the bank that issued it, its branch etc.
hexadecimal
0 - 9 plus A - F
Hexadecimal is commonly used in comoputing to represent a memory byte.
A letter used to represent a number is a variable of sometimes a constant.
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Without context, it is difficult to say. I guess you are referring to number bases greater than 10 where letters are used to represent digits above 9; the first digit would be A, the second B and so on. In these cases the letter A is being used to represent 10 times the column value where it is. For example in hexadecimal, the A in 0x6A3 represents 10 x 16 = 160 in decimal (since it is in the sixteens column of the number).
Variable
A letter used to represent a number is called a variable. Typically, the letters x and y are used, but technically you can use any letter of the alphabet.
These are used quite often to represent values stored in bytes - 1 byte is represented as two hexadecimal digits. For example, both the MAC address of a network card and the new IP addresses (IPv6) are usually shown as hexadecimal.
A variablẹ
A variable
a variable