The number of positive integers with exactly four decimal digits that is positive integers between 1000 and 9999 is 4536.
The positive integers between 1000 and 9999 inclusive have 4 digits.
The first digit cannot be zero because otherwise, it will become a digit integer.
So, the first digit can be filled in 9 ways.
The second digit can be filled by 9 different ways.
Because the second digit cannot be the same as that of the first digit but zero can be the second digit.
The third digit can be filled by 8 different ways.
The fourth digit can be filled in 7 different ways.
Number of four decimals digits = 9 * 9 * 8 * 7
= 81 * 8 * 7
= 81 * 56
= 4536 digits.
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