1 to 9

Call a positive integer complete if it uses every digit from 1 to 9 once. Examples of complete numbers are 192837465, 987654321 and 491857263.

What is the sum of all complete numbers?


The answer is 201599999798400.

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2 solutions

Each digit appears in each column 8 ! = 40320 8!=40320 times.

So each column must add up to 40320 ( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ) = 181400 40320(1+2+3+4+5+6+7+8+9)=181400

Therefore, the sum of all complete numbers is

s u m = 1814400 ( 1 + 10 + 100 + 1000 + 10000 + 100000 + 1000000 + 10000000 + 100000000 ) = sum=1814400(1+10+100+1000+10000+100000+1000000+10000000+100000000)= 201599999798400 \large \color{#D61F06}\boxed{201599999798400}

@Vu Vincent

  1. When naming terms, try and match it up with the description. For example, in this case, calling a number "ordered" would suggest that the digits are ordered in some way. As such, I replaced it with "complete", indicating that it has all of those digits.

  2. When giving examples, try and give examples that are representative of the set that you're describing, and not just focused on a subset. For example, when describing animals, it is not ideal to just say "tigers, leopards, lions", since someone who didn't read the term might think we're talking about "wild cats" only. As such, I gave examples where the digits were not ordered nicely.

Calvin Lin Staff - 3 years, 10 months ago
Vu Vincent
Jul 25, 2017

There exists a complete number that adds up to another complete number such that they equal to 1111111110 1111111110

Consider the complete number 123456789 123456789 . Now, observe that 987654321 987654321 "matches" with each digit in 123456789 123456789 . Taking their sum lends us 1111111110 1111111110 . Another example would be 967485231 967485231 which pairs up with 143625879 143625879 and adds up to 1111111110 1111111110 . So, there will always be 2 complete numbers such that they sums up to 1111111110 1111111110 .

There are 9 ! 9! complete numbers in total, hence there are exactly 9 ! 2 \frac{9!}{2} pairs, each having a value of 1111111110 1111111110

Therefore, the sum of all complete number is:

1111111110 × 9 ! 2 = 201599999798400 1111111110 \times \frac{9!}{2} = \large \boxed{201599999798400}

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