I have researched and found 2 approaches but haven't understood both.Can anyone explain it clearly or probably with any real world example?
Four digit numbers $ = 4 \cdot 3 \cdot 2 \cdot 1 = 24$ ways we can form a four digit number. Since it's a 4 digit number, each digit will appear $6= 24/4$ times in each of units, tens, hundreds, and thousands place. Therefore, the sum of digits in the units place is $6(1+2+5+6)=84$.
Why they are adding $1+2+5+6$?
Similarly sum of digits at tens,hundreds,thousands place is $84$
Required sum of all numbers $= 84+ 84 \cdot 10+ 84 \cdot 100+ 84 \cdot 1000=93324$
I found it here:
Though I have understood the 2 approaches, I am not able to understand the question.