Given you have a four digit house number what are the chances that you could make a given 3 digit house number from the numbers you all ready had. In my case the number was 711.

I wrote a program to figure out the combinations of 711 in four digit numbers and came up with 106. However numbers like 0711 0171 0117 would not be applicable as no one put 0117 on there house if there address is 117.

So I came up with 103/8999 how ever I could not come up with a general formula to model the problem as there are more combinations of numbers with out a repeating digit.