Problem solving - 10

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

Answer -- C
The sum of the units is 40. Then add 4 to the tens , whose sum is 39 and 3 is 43 which gives 3 as tens digit