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# M11 #1

Author Message
Manager
Joined: 25 May 2009
Posts: 144
Concentration: Finance
GMAT Date: 12-16-2011
Followers: 1

Kudos [?]: 69 [0], given: 2

M11 #1 [#permalink]  30 Jun 2009, 18:23
What is $$1 + 2 + ... + 98$$ ?
A 4750
B 4763
C 4790
D 4801
E 4851

[Reveal] Spoiler:
OA=E
Manager
Joined: 30 Nov 2008
Posts: 91
Followers: 2

Kudos [?]: 18 [0], given: 0

Re: M11 #1 [#permalink]  01 Jul 2009, 08:40
I got E

98+1=99, 97+2=99, 96+3.......There are 49 of these additions (98/2) so:

49*99=4851
Director
Joined: 04 Jan 2008
Posts: 914
Followers: 58

Kudos [?]: 327 [0], given: 17

Re: M11 #1 [#permalink]  01 Jul 2009, 08:55
Its AP with a=1,d=1,n=98

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

its E
_________________
Manager
Joined: 25 May 2009
Posts: 144
Concentration: Finance
GMAT Date: 12-16-2011
Followers: 1

Kudos [?]: 69 [0], given: 2

Re: M11 #1 [#permalink]  02 Jul 2009, 03:50
Figured out another way:

S = (n(Last+First))/2
n= (First-Last) + 1

n=98
S=(98(99))/2 = 4851 so E.
Re: M11 #1   [#permalink] 02 Jul 2009, 03:50
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# M11 #1

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