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# number of games played

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Senior Manager
Joined: 10 Nov 2010
Posts: 268
Location: India
Concentration: Strategy, Operations
GMAT 1: 520 Q42 V19
GMAT 2: 540 Q44 V21
WE: Information Technology (Computer Software)
Followers: 5

Kudos [?]: 47 [0], given: 22

number of games played [#permalink]  20 Feb 2011, 08:13
00:00

Difficulty:

(N/A)

Question Stats:

100% (01:35) correct 0% (00:00) wrong based on 3 sessions
sol

8th team will play 7 games with 7 teams, so total number of games played by 8th team is 7
similarly 7----------------6------------6------------------------------------------ 7----------- 6
--------6----------------5------------5------------------------------------------ 8----------- 5
nd so on

total games played= 7+6+5+4+3+2+1=28

any other optimal sol.
[Reveal] Spoiler: OA

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The proof of understanding is the ability to explain it.

Math Forum Moderator
Joined: 20 Dec 2010
Posts: 2047
Followers: 128

Kudos [?]: 904 [1] , given: 376

Re: number of games played [#permalink]  21 Feb 2011, 10:48
1
KUDOS
144144 wrote:
in what cases u will use the formula n(n+1)/2?

Just to add first n natural numbers; that's all

1+2+3+4+5
Here n=5; Sum = n(n+1)/2 = 5*6/2=15

1+2+3+4+5+6+7+8+9
Here n=5; Sum = n(n+1)/2 = 9*10/2=45

In the above example;
7+6+5+4+3+2+1
Sum of natural numbers from 1 to 7 = 7*8/2 = 56/2 = 28.

It easy this way rather than adding every number.
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Math Forum Moderator
Joined: 20 Dec 2010
Posts: 2047
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Kudos [?]: 904 [0], given: 376

Re: number of games played [#permalink]  20 Feb 2011, 08:17
Number of ways 2 teams can be selected out of 8 teams:

C^8_2=\frac{8*7}{2}=28

Ans:"C"
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Kudos [?]: 4875 [0], given: 163

Re: number of games played [#permalink]  20 Feb 2011, 19:20
Expert's post
GMATD11 wrote:
sol

8th team will play 7 games with 7 teams, so total number of games played by 8th team is 7
similarly 7----------------6------------6------------------------------------------ 7----------- 6
--------6----------------5------------5------------------------------------------ 8----------- 5
nd so on

total games played= 7+6+5+4+3+2+1=28

any other optimal sol.

Your reasoning is perfect.... Just that to perform 7+6+5+4+3+2+1, use the formula n(n+1)/2 where n = 7.
7*8/2 = 28
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Senior Manager
Joined: 08 Nov 2010
Posts: 422
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Kudos [?]: 34 [0], given: 161

Re: number of games played [#permalink]  21 Feb 2011, 10:41
in what cases u will use the formula n(n+1)/2?
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Re: number of games played   [#permalink] 21 Feb 2011, 10:41
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