- #1

- 19

- 0

**Is there a formula?**

## Homework Statement

Determine the sum of all the four digit numbers formed by using the digits 1,3,5, and 7. NO DIGIT REPEATED in any number.

## Homework Equations

thats my question...

## The Attempt at a Solution

I can solve this problem by simply writing out all of the terms and adding them up...

ie

1357+1375+...3157+3175+...+ 5137+5173+...+7135+7153+...7531 =:zzz:

But is there a formla which I can use for these types of questions? For example if there were 9 different numbers I would not want to write out all the combinations and then add up all the numbers. It wasnt to difficult with just 4 but, I knowthere must be some formula or theorem out there... Any ideas?