Hi :))
I'm going to group the numbers to make them easy to calulate.
i = 10, 20, 30, ..., 90. 1 + 2 + 3 + ... + 9
1 <= i <= 9. 1 + 2 + 3 + ... + 9
11 <= i <= 19. 1 + 2 + 3 + ... + 9
21 <= i <= 29. 2(1 + 2 + 3 + ... + 9)
31 <= i <= 39. 3(1 + 2 + 3 + ... + 9)
...
91 <= i <= 99. 9(1 + 2 + 3 + ... + 9)
Overall. there are 1 + 1 + 1 + 2 + 3 + ... + 9 = 2 + 10*9/2 = 47 groups of (1 + 2 + 3 + ... + 9)
47*(1 + 2 + 3 + ... + 9) = 47*45 = 2115.
I hope this helped. :))
=^._.^=