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A sequence 1, 2, 3, 4. What is the tens' digit of the sum of

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A sequence 1, 2, 3, 4. What is the tens' digit of the sum of [#permalink] New post 09 Aug 2006, 01:47
A sequence 1, 2, 3, 4. What is the tens' digit of the sum of first forty terms?
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 [#permalink] New post 09 Aug 2006, 05:17
sum = n(n+1)/2 = 40*41/2 = 820
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 [#permalink] New post 13 Aug 2006, 11:49
agree with AK 2 it is !
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Re: Sequence [#permalink] New post 13 Aug 2006, 17:22
getzgetzu wrote:
A sequence 1, 2, 3, 4. What is the tens' digit of the sum of first forty terms?


is the Q 1+2+3+4...+38+39+40 ??

Arithmetic progression = 20[2+39]
= 20*41
= 820

Hence tens digit =2

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 [#permalink] New post 13 Aug 2006, 18:01
for n consecutive integers,
sum is n(n+1)/2
for 40 integers consecutive - 40*41/2=820

Tens digit 2
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 [#permalink] New post 15 Aug 2006, 03:58
the ten digit is 2.

Sum of first n number is n( n+1) /2 = 40 ( 41) /2 = 820

tenth Digit is 2
  [#permalink] 15 Aug 2006, 03:58
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A sequence 1, 2, 3, 4. What is the tens' digit of the sum of

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