Take a number say 987654.
Sum it's digits
9 + 8 + 7 + 6 + 5 + 4 = 39
3 + 9 = 12
1 + 2 = 3
i.e. keep doing this till you get a single digit answer.
Now I take the same number & do it in other different ways, I still end up with the same answer.
987 + 654 = 1641
16 + 4 + 1 = 21
2 + 1 = 3
Or
98765 + 4 = 98769
9876 + 9 = 9885
988 + 5 = 993
99 + 3 = 102
1 + 0 + 2 = 3
How come I always get the same answer (3 in this case). This is not special for 987654. It's for any number you take.
What's the reason or theory behind this?
(PS - I am not sure what's the right tag for this question. Please correct if necessary).