The question assumes 475.641 is not a whole number. The dot separates the fractional part from the integer part. Converting to binary you get $|100|111|101.110|100|001$. Now you convert to hex, starting from the decimal point, so the first step is to regroup the digits into groups of four: $0001|0011|1101.1101|0000|1000$. Now you convert to hex digits to get $13D.D08$.
The question assumes 475.641 is not a whole number. The dot separates the fractional part from the integer part. Converting to binary you get $|100|111|101.110|100|001$. Now you convert to hex, starting from the decimal point, so the first step is to regroup the digits into groups of four: $0001|0011|1101.1101|0000|1000$. Now you convert to hex digits to get $13D.D08$.