Learn in the textbookBinary Numbers and Radix
DefinitionRadix conversion (基数変換) means rewriting a value in a notation with a different radix while leaving the value itself unchanged.
ExplanationTo turn a binary number into decimal, multiply each digit by its weight and add the results. To turn a decimal number into binary, keep dividing by 2 until the quotient reaches 0, then line up the remainders starting from the last one. Hexadecimal uses 16 digits, 0 to 9 and A to F; one hexadecimal digit matches four binary digits, and one octal digit matches three.
AnalogyLike restating the same length in inches instead of centimeters: the amount stays put, and only the ruler used to write the number changes.
Memory hook16 is 2 to the 4th power, so one hex digit covers four binary digits; 8 is 2 to the 3rd power, so one octal digit covers three.