GMAT Prep Test 2

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dsuna: Junior | Next Rank: 30 Posts; Posts: 14; Joined: Sat Mar 17, 2007 7:35 pm

by dsuna » Sun Mar 25, 2007 9:26 am

If eleven consecutive integers are listed from least to greatest, what is the average (arithmetic mean) of the eleven integers?

1.The average of the first nine integers is 7
2. The average of the last nine integers is 9

Ans. D

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Source: Beat The GMAT — Data Sufficiency | Sun Mar 25, 2007 9:26 am

jayhawk2001: Community Manager; Posts: 789; Joined: Sun Jan 28, 2007 3:51 pmFollowed by: 1 member; Location: Silicon valley, California; Thanked: 30 times

by jayhawk2001 » Sun Mar 25, 2007 12:07 pm

Sum of series a1, a2...an = n/2 (2a + (n-1)d)
Average = 1/2 (2a + (n-1)d)

1 - sufficient.
1/2 (2a1 + 8) = 7. We can find a1 and hence we can find average of
11 consecutive integers starting with a1

2 - sufficient. Same as 1

Hence D.

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