One fewer than the base for the number system.
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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.
Zero is itself a digit so the question does not really make sense. However, the non-zero digits are older than zero. For example, consider the Roman number system which had no zero.
100 is a 3 digit number.