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What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851

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CEO
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What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851   [#permalink]

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New post 28 Oct 2007, 13:09
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What is 1 + 2 + 3 ...98?

4750
4763
4790
4801
4851

What is the logic of your approach towards this question?

--== Message from GMAT Club Team ==--

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Director
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Posts: 889
Re: PS Counting  [#permalink]

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New post 28 Oct 2007, 13:23
bmwhype2 wrote:
What is 1 + 2 + 3 ...98?

4750
4763
4790
4801
4851

What is the logic of your approach towards this question?


i get 4851 using the summation formula for arithmetic progression

Sn = n/2 (2a + (n-1) d

n = # of terms = 98
a = value of first term = 1
d = difference between terms = 1 (since consecutive)

Sn = 98/2 ( 2*1 + (98-1)(1))
= 49 (2+97) = 4851
VP
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New post 28 Oct 2007, 13:24
(98*99)/2 = 4,851

This is true for every 1+2+3 ... n

sum =(n*(n+1))/2

:)
Director
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Re: PS Counting  [#permalink]

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New post 28 Oct 2007, 13:26
bmwhype2 wrote:
What is 1 + 2 + 3 ...98?

4750
4763
4790
4801
4851

What is the logic of your approach towards this question?


My logic is to remember formula for arithmetic progression.

Sn = n/2 [ 2a + (n-1) d]

= 98 * 99 /2 = 4851
Manager
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Joined: 02 Aug 2007
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New post 28 Oct 2007, 15:50
Does anyone has any practice problems testing this forumla?
Director
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New post 28 Oct 2007, 15:53
1
Check the PR book. This is a pretty standard GMAT formula you should know for the test. And to break it down in simple terms:

(How many #s in set/2) TIMES (First Number in set + Last Number in set) so..


(98/2) * (99) = 4851

Things to watch out for is the question...how many #s in the set. It just so happens we started with 1 in this case. But what if I said 13 to 98, inclusive. How many integers then...answer?
Manager
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New post 28 Oct 2007, 16:02
13-98 is the range with d = 1

Sum of numbers in this sequence:
= 4675
Manager
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New post 28 Oct 2007, 16:13
another way to solve it is:
1+98 = 99
2+97 = 99
3+96 = 99

so you will have 49 values that are all equal to 99
so 49*99 = 4851
CEO
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Joined: 21 Jan 2007
Posts: 2685
Location: New York City
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New post 28 Oct 2007, 23:00
jimjohn wrote:
another way to solve it is:
1+98 = 99
2+97 = 99
3+96 = 99

so you will have 49 values that are all equal to 99
so 49*99 = 4851


this is the way i solved this.

OA is E.
GMAT Club Legend
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Joined: 07 Jul 2004
Posts: 4939
Location: Singapore
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New post 29 Oct 2007, 01:35
This is an arithmetric progression where first term = 1, last term = 98, number of terms = 98. Then sum of this progression = 98/2 (1 + 98) = 4851

ans D
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Re: What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851   [#permalink]

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New post 21 Mar 2017, 20:23
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Re: What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851 &nbs [#permalink] 21 Mar 2017, 20:23
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