Official Solution:
The consumer price index gives us a ratio between prices in 2000 and prices in 2009. We are told that "for every Zeropian dollar spent on consumer goods in 2000, $1.75 on average had to be spent in 2009." In other words, if something cost \(X\) dollars in 2000, it cost \(1.75 \times X\) dollars in 2009 (as long as the price increased exactly according to the index, which is just an average). In dollar terms, the increase in price would then be \(1.75 \times X - X = 0.75 \times X\) dollars.
We are asked for this dollar price increase for Brand Z running shoes. Representing the price of these shoes in 2000 as \(X\), as we already have, we can rephrase the question as "What is \(0.75 \times X\)?" We can further rephrase this question to "What is \(X\)?"
(1) SUFFICIENT. We are told that the price of the shoes in 2009 is $91. We have represented the 2009 price as \(1.75 \times X\) dollars, staying consistent with our variable naming throughout the problem (never change variable designations midstream unless you’re starting over completely). So we can write an equation:
\(1.75 \times X = 91\)
We know we can solve for \(X\), so we can answer the question. (Incidentally, if we had to solve for this \(X\) on a Problem-Solving problem, one fast way would be to convert 1.75 to a fraction. \(1.75 = \frac{7}{4}\), so we can quickly write that \(X = 91 \times \frac{4}{7}\). Since \(\frac{91}{7} = 13\), we get \(X = 13 \times 4 = 52\).)
(2) INSUFFICIENT. We are told that the price increase in dollar terms, divided by the price of the shoes in 2009, is \(\frac{3}{7}\). However, this information is already completely implied by the stem. If the index is 1.75, then any good’s price increase was 75%, or 75 cents for every 2000 dollar. Since the 2009 price is $1.75 for every 2000 dollar, the ratio of the price increase ($0.75) to the 2009 price ($1.75) will always be \(\frac{0.75}{1.75}\), or \(\frac{3}{7}\). This holds true no matter what the original 2000 price is, so we cannot determine \(X\) through this bit of redundant information.
Answer: A