- #1

- 19

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**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?