What is the tens' digit of the sum of the first 40 terms of

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What is the tens' digit of the sum of the first 40 terms of [#permalink]

26 Jan 2012, 07:28

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What is the tens' digit of the sum of the first 40 terms of 1, 11, 111, 1111, 11111, 111111, ...?

A. 2
B. 3
C. 4
D. 8
E. 9

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Re: tens' digit [#permalink]

26 Jan 2012, 09:19

LM wrote:

What is the tens' digit of the sum of the first 40 terms of 1,11,111,1111,11111,111111,.......?

A)2
B)3
C)4
D)8
E)9

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

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Re: tens' digit [#permalink]

26 Jan 2012, 09:50

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LM wrote:

What is the tens' digit of the sum of the first 40 terms of 1,11,111,1111,11111,111111,.......?

A. 2
B. 3
C. 4
D. 8
E. 9

----1
---11
--111
-1111
11111
......

For the units digit we should add 40 1's --> 40*1=40, so 0 will be units digit and 4 will be carried over;

For the tens digit we should add 39 1's --> 39*1=39+4=43, so 3 will be the tens digit.

Answer: B.

Similar question: infinite-seq-of-97477.html

Hope it helps.
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Re: What is the tens' digit of the sum of the first 40 terms of [#permalink]

21 Dec 2012, 07:53

LM wrote:

What is the tens' digit of the sum of the first 40 terms of 1, 11, 111, 1111, 11111, 111111, ...?

A. 2
B. 3
C. 4
D. 8
E. 9

40*1 (units digits) = 40, carry over 4
39*1 + 4 = 43, carry over 4

Answer: B

Re: What is the tens' digit of the sum of the first 40 terms of [#permalink] 21 Dec 2012, 07:53

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What is the tens' digit of the sum of the first 40 terms of

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What is the tens' digit of the sum of the first 40 terms of

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