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Xbond: Master | Next Rank: 500 Posts; Posts: 275; Joined: Wed Jul 02, 2008 4:19 am; Thanked: 4 times

by Xbond » Mon Jun 08, 2009 1:11 pm

Hi there,

Someone can help to understand the concept. I hesitate between C and E
Who can demonstrate for sure the right answer ?

If the average (arithmetic mean) of four different numbers is 30, how many of the numbers are greater than 30 ?

(1) None of the four numbers is greater than 60

(2) Two of the four numbers are 9 and 10, respectively

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Source: Beat The GMAT — Data Sufficiency | Mon Jun 08, 2009 1:11 pm

abhinav85: Master | Next Rank: 500 Posts; Posts: 179; Joined: Mon Dec 29, 2008 4:41 am; Thanked: 2 times

by abhinav85 » Mon Jun 08, 2009 1:17 pm

IMO C

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Xbond: Master | Next Rank: 500 Posts; Posts: 275; Joined: Wed Jul 02, 2008 4:19 am; Thanked: 4 times

by Xbond » Mon Jun 08, 2009 1:27 pm

ok, the purpose of my question is to get a clear demonstration not only the answer.

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tohellandback: Legendary Member; Posts: 752; Joined: Sun May 17, 2009 11:04 pm; Location: Tokyo; Thanked: 81 times; GMAT Score:680

by tohellandback » Mon Jun 08, 2009 10:20 pm

Xbond:
here is my explanation with examples
Case 1:insufficient
consider these set of numbers:

28,29,31,32
28,29,30,33

you see you can't tell how many are greater than 30
Case 2:

9,10,50,51
9,10,1,100
you still can't say

Combine both
9,10, the third number lets say is 30. Now to have an average of 30 you must have the 4th number 61 which violates the condition in 1. so the third number has to be at least 31

gotcha!!:))
its C. there are two numbers greater than 30

The powers of two are bloody impolite!!

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Xbond: Master | Next Rank: 500 Posts; Posts: 275; Joined: Wed Jul 02, 2008 4:19 am; Thanked: 4 times

by Xbond » Tue Jun 09, 2009 11:58 am

many thks

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