What is the tens' digit T of the sum of the first 40 terms of 1, 11, 111, 1111, 11111, 111111, ...?
What is the tens' digit T of the sum of the first 40 terms of 1, 11, 111, 1111, 11111, 111111, ...?
Answer/Solution
T=3
Steps/Work
All of the first 40 terms have 1 at the units place.
Except the first term, the tens digit of all the remaining 39 terms is 1.
So now if you do the addition
1 1
1 1
. . .
. . .
. . .
1 1
0 1
3 0
Answer is B
Except the first term, the tens digit of all the remaining 39 terms is 1.
So now if you do the addition
1 1
1 1
. . .
. . .
. . .
1 1
0 1
3 0
Answer is B