What is the binary equivalent of the decimal number 85.125?

The decimal 85.125 converts to binary as 1010101.001.

The integer part 85 in binary is 1010101, from 64+16+4+1.

The fraction 0.125 equals one eighth, written as 0.001 in binary.

Joining the two parts gives 1010101.001.

Values like 1001001.111 or 1100101.001 have the wrong integer bits for 85.

Each binary place after the point represents a half, a quarter, an eighth and so on.