SOLUTION: Suppose you select a 3-digit number at random from the set of all positive 3-digit numbers. Find each probability. I just don't get this.

The three digit numbers are the ones from 100 through 999 inclusive.

The problem is really like this:

Suppose there are a lot of cards in a box numbered from 100-999.  
Somebody says "Draw out card number (say) 731".  What is the probability
that he will reach in there blindly among those and actually draw card
number 731 out of the box?

First we answer the question: How many of those cards are there in that
bos?  Well there are 999 integers between 1 and 999, but the 99 
integers from 1 through 99 have less than 3 digits, so we must subtract
those 99 from 999. Therefore there are 999 - 99 = 900 3 digit numbers. 

He hopes to draw card number 731.  However he has just 1 chance in 900
of drawing it. So that means the probability is 1 out of 900 or .

(It doesn't have to be 731. It can be any 3-digit number, I just arbitrarily 
picked 731 as an illustration).

Edwin