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?
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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.
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.
There exists a complete number that adds up to another complete number such that they equal to 1 1 1 1 1 1 1 1 1 0
Consider the complete number 1 2 3 4 5 6 7 8 9 . Now, observe that 9 8 7 6 5 4 3 2 1 "matches" with each digit in 1 2 3 4 5 6 7 8 9 . Taking their sum lends us 1 1 1 1 1 1 1 1 1 0 . Another example would be 9 6 7 4 8 5 2 3 1 which pairs up with 1 4 3 6 2 5 8 7 9 and adds up to 1 1 1 1 1 1 1 1 1 0 . So, there will always be 2 complete numbers such that they sums up to 1 1 1 1 1 1 1 1 1 0 .
There are 9 ! complete numbers in total, hence there are exactly 2 9 ! pairs, each having a value of 1 1 1 1 1 1 1 1 1 0
Therefore, the sum of all complete number is:
1 1 1 1 1 1 1 1 1 0 × 2 9 ! = 2 0 1 5 9 9 9 9 9 7 9 8 4 0 0
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Each digit appears in each column 8 ! = 4 0 3 2 0 times.
So each column must add up to 4 0 3 2 0 ( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ) = 1 8 1 4 0 0
Therefore, the sum of all complete numbers is
s u m = 1 8 1 4 4 0 0 ( 1 + 1 0 + 1 0 0 + 1 0 0 0 + 1 0 0 0 0 + 1 0 0 0 0 0 + 1 0 0 0 0 0 0 + 1 0 0 0 0 0 0 0 + 1 0 0 0 0 0 0 0 0 ) = 2 0 1 5 9 9 9 9 9 7 9 8 4 0 0