There are 21.
Three of them.
50%
There are 20.
A bijective numeration is a numeral system which uses digits to establish a bijection between finite strings and the positive integers.
20. 16 without repeating a digit.
Three of them.
81
The sum of all the digits of all the positive integers that are less than 100 is 4,950.
279,999,720
50%
They are 13.
405
52
There are none.
Every number from 100 to 999 inclusive !
3
The sum of the digits is 6.