Imagine writing all these numbers on top of each other, in columns, the 'units' column (the far right), every 10'th number is the same, so you have continuing cycles of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
So in this column, the sum is \frac{1000000}{10} ( 1 + 2 + \dots + 9)
The next column, the...