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.
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The number zero was the first "invented" number.
The smallest four-digit number not using zero is 1000. In a four-digit number, the first digit cannot be zero, so the smallest possible value for the first digit is 1. The remaining three digits can be any number from 0-9, so the smallest possible four-digit number is 1000.
5. Count the number of digits from the first non-zero digit to the last non-zero digit.
The key in this type of problems is to advance from left to right, and to use the smallest possible digit in each case. Thus, the first digit (from the left) has to be a one, since a number doesn't start with a zero; the next digit has to be a zero; etc.
6210001000