Number Systems (1.3.a)
The relationship between a digit, its position, and the base of the number system is expressed by a simple formula:
DIGIT * BASE POSITION
Download a free excel number base calculator which works for up to base-36, based on the above equation.
Octal is base-8. You can convert from binary to octal and back without "going through" decimal, by converting 3 binary bits to 1 octal bit, because 3 binary bits can represent up-to decimal 7, as can 1 octal bit.
Hexadecimal is base-16. Like with octal, you can convert from binary to hexadecimal (and back), by using 1 hexadecimal bit to represent 4 binary bits.
Binary-Coded Decimal (BCD)
Binary-Coded Decimal (BCD) is when the individual decimal digits are represented using 4 binary bits.