Answer:
10090
Step-by-step explanation:
There are 9 one-digit positive integers (1 through 9). There are 900 three-digit positive integers, since there are 100 three-digit numbers with each hundreds digit from 1 to 9. So far, we've counted 909 positive integers with an odd number of digits. Next are five-digit numbers. Since 909 + 91=1000, we're looking for the 91st five-digit number.
The first five-digit number is 10000, so the 91st five-digit number is 10000 + 90, or 10090.