There will be 120 numbers.

On ones digit place(when all numbers are listed for addition) ,

 there will be 24 5's , 24 4's, 24 3's , 24 2's, 24 1's .

There sum will be 24(15)=360

Now, this will repeat for ten's place too and further...

 

                   No.s

                    No.s

                    No.s

                   No.s

                      .       

                      .

                      .

                      .

             -------------------------

Sum            3999960  

As u know , the sum of digits at every place (360), u can carry 36 to the next place and find the sum by normal addition.

 

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