Explanation:
the question is a little bit misleading, as there is no single number that is 100 more than 408 AND 100 more than 829 AND 100 more than 973.
so, all we can do is find the 3 numbers that are 100 more than the given 3 numbers.
408
100
----------
508
we go through from the right to the left adding each pair of digits : 0+8 = 8, 0+0 = 0, 1+4 = 5
829
100
-----------
929
as before, 0+9 = 9, 0+2 = 2, 1+8 = 9
973
100
-----------
1073
as before, 0+3 = 3, 0+7 = 7, 1+9 = 10.
here we are "lucky" that this happened with the last pair of digits. so, we have the space to write 10 (two digits instead of only one).
what do we do, when this happens on the middle of a number ? we "carry" the "1" of the 10 (or 11 or 12 ...) into the sum of the next pair.
e.g.
478
253
------------
731
3+8 = 11, so, we write the right "1" and carry the left "1" into the next.
5+7+1 = 13, so, we write the right "3" and carry the left "1" into the next.
2+4+1 = 7. done.