A infinite sequence is 1, 11, 111, 1111, 11111, ..., what is the tens digit of the sum of the first 40 terms?
A) 1
B) 2
C) 3
D) 4
E) 9
OA = C
A) 1
B) 2
C) 3
D) 4
E) 9
OA = C
Look at it this way: no matter how many 1s you have in a number of the sequence, it will always end in 11 starting with the second number in the sequence. This is the key to the problem.
Now, this means that the tens digit of 1 + 11 + 111 + ... will be the tens digit of 1 + 39 * 11 (since the tens digit cannot be affected by any other digits apart from units and tens). You have that "1" over there because, as I've pointed out, the first number in the series is 1 and not 11.
1 + 39 * 11 = 1 + 39 * 10 + 39 = 40 + 390 = 430 - tens digit 3.
