GMAT PREP difficult problem

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rdchandvadkar: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Tue Sep 22, 2009 8:25 pm

by rdchandvadkar » Mon Feb 01, 2010 11:13 pm

The cost of delivery for an order of desk chairs was $10.00 for the 1st chair, and $1 for each additional chair in the order. If an office manager placed an order for n desk chairs, is n>24?

(1) The delivery cost for the order totaled more than $30.00
(2) The average (arithmetic mean) delivery cost per chair of the n chairs was $1.36

Answer: B

please help. got the answer but need to know if my reasoning was correct.

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Source: Beat The GMAT — Data Sufficiency | Mon Feb 01, 2010 11:13 pm

ajith: Legendary Member; Posts: 1275; Joined: Thu Sep 21, 2006 11:13 pmFollowed by: 2 members; Location: Arabian Sea; Thanked: 125 times

by ajith » Tue Feb 02, 2010 12:13 am

rdchandvadkar wrote:The cost of delivery for an order of desk chairs was $10.00 for the 1st chair, and $1 for each additional chair in the order. If an office manager placed an order for n desk chairs, is n>24?

(1) The delivery cost for the order totaled more than $30.00
(2) The average (arithmetic mean) delivery cost per chair of the n chairs was $1.36

Answer: B

please help. got the answer but need to know if my reasoning was correct.

1) First chair costs 10; each of the next chair costs 1 so if the total cost is over 30,

He must have placed orders for at least 1+ (30-10)/1 = 21 that is no way sufficient to conclude that n>24 or not

2) say they ordered n chairs cost of (n-1) of them is (n-1) in total and cost of first chair is 10

so total cost = n-1+10 = n+9

now Mean = Total cost/ Total no of chairs = n+9/n =1.36

solving we will get an n either over or under 24 which will enable us to answer the question, Sufficient

B

Always borrow money from a pessimist, he doesn't expect to be paid back.

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sunil_snath: Senior | Next Rank: 100 Posts; Posts: 39; Joined: Wed Nov 18, 2009 8:41 am; Thanked: 3 times

by sunil_snath » Tue Feb 02, 2010 1:16 am

STMT1: Delivery cost > 30
10 + (n-1) > 30
n > 21
INSUFF

STMT2: AM = 1.36
10 + (n-1) = 1.36n
Value of n can be found: SUFF

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