Data Sufficiency

BTGmoderatorDC wrote:The cost of delivery for an order of desk chairs was $10.00 for the first chair, and $1.00 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 totalled more than $30.00.
(2) The average (arithmetic mean) delivery cost of the n chairs was $1.36.

OA B

Source: GMAT Prep

Target question: Is n > 24?

Given: 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. The office manager placed an order for n desk chairs.

Statement 1: The delivery cost for the order totaled more than $30.00
There are several scenarios that meet this condition. Here are two:
Case a: n = 25, in which case the total cost is $34 (which is more than $30). Here, n is greater than 24.
Case b: n = 23, in which case the total cost is $32 (which is more than $30). Here, n is not greater than 24.
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: The average (arithmetic mean) delivery cost per chair of the n chairs was $1.36
The average cost of n chairs = (total cost of n chairs)/n
The total cost = $10 for the first chair plus $1 for each of the remaining n-1 chairs
Algebraically, we can say that the total cost = 10 + 1(n-1)
So, the average cost of n chairs = [10 + 1(n-1)]/n
Statement 2 says the average cost is $1.36, so we can write:
[10 + 1(n-1)]/n = 1.36
IMPORTANT: We could solve this equation for n (but we won't), which means we could definitively determine whether or not n > 24.
Since we could answer the target question with certainty, statement 2 is SUFFICIENT

Answer = B

Cheers,
Brent