One fewer than the base for the number system.
The binary system uses two digits, zero and one.
5. Count the number of digits from the first non-zero digit to the last non-zero digit.
All numbers which are not "zero" are classed as significant digits. Therefore in the number 129 there will be three digits which are classed as significant.
100 is a 3 digit number.
5 bits are 5 binary digits. If they represent a decimal number, then that number can be anything from zero to 31, and can have either 1 or 2 digits.