Four digit numbers =4⋅3⋅2⋅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?
0:453:22Finding the Sum of numbers – YouTubeYouTube