1. ## Easy combinatorics problem

How many four-digit positive integers are there that contain the digit 1?

2. ## Re: Easy combinatorics problem

Originally Posted by alexmahone
How many four-digit positive integers are there that contain the digit 1?
To be clear $0109=109$ is not a four digit number, right??
There are $9\cdot 10^3$ four-digit numbers.
Of those there are $8\cdot 9^3$ that do not contain any 1's.

So how many contain at least one 1?

3. ## Re: Easy combinatorics problem

hi alexmahone

should the digits be the same or must be different?
if they are the same then there are:
1000+900+900+900=2800

if they are not the same then there are:
720+648+648+648=2664