Any number system has a base.
Decimal = Base 10 i.e. the "ones" place consists of 10^0 power. while the tens place is 10^1
Octal then = Base 8 i.e. the "ones" place consists of 8^0 (0-7) and the tens (eights) place is 8^(1)
Hex = Base 16 the "ones" place consists of 16^0 and the tens (sixteens) place is 16^(1)
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Binary to Decimal :-
Binary :-10001011
Decimal is :-1(2^7)+0(2^6)+0(2^5)+0(2^4)+1(2^3)+0(2^2)+1(2^1)+1(2^0)
1(128)+0+0+0+1(8)+0+1(2)+1(1)
128+8+2+1
139
Octal:-213
Decimal is :-2(8^2)+1(8^1)+3(8^0)
2(64)+1(8)+3(1)
128+8+3
139
Hexa-Decimal :-8B
Decimal is :- 8(16^1)+B(16^0)
8(16)+11(1)
128+11
139
In hexadecimal there is 16 digits 0 to 15 but digits are always singals so there is numbers 0 to 9 and
then comes 10 but 10 has 2digits so 10 becomes A then 11 becomes B 12 becomes C 13 becomes D 14 becomes E and 15 becomes F.
So hexa decimal has 16 numbers :-0 TO 9,A,B,C,D,E,F.