Extra information given in the question stem.

[ashforgmat]

The ratio of boys to girls in Class A is 3 to 4. The ratio of boys to girls in Class B is 4 to 5. If the two classes were combined, the ratio of boys to girls in the combined class would be 17 to 22. If Class A has one more boy and two more girls than class B, how many girls are in Class A?

8
9
10
11
12

Now in the official answer explanation the information "If the two classes were combined, the ratio of boys to girls in the combined class would be 17 to 22" is not used et all.
Can this happen on the GMAT? That some info. given in the question is not et all used?

[towerSpider]

Nice question because sometime you would not be able to decode the method and then you would ask yourself: "why the the heck would the question-maker give this particular info?" ;- )

[GMATGuruNY]

The ratio of boys to girls in Class A is 3 to 4. The ratio of boys to girls in Class B is 4 to 5. If the two classes were combined, the ratio of boys to girls in the combined class would be 17 to 22. If Class A has one more boy and two more girls than class B, how many girls are in Class A?

8
9
10
11
12

Now in the official answer explanation the information "If the two classes were combined, the ratio of boys to girls in the combined class would be 17 to 22" is not used et all.
Can this happen on the GMAT? That some info. given in the question is not et all used?

I've seen a few GMAT problems that offer unnecessary information. Such a situation is rare, however. In my BTG article How to Unstick Yourself (https://www.beatthegmat.com/mba/2010/07/ ... k-yourself), I suggest that if you're ever stuck on a math problem, you should ask yourself what pieces of information you haven't used. Usually any information given will be needed in order to answer the question.

By the way, the problem above is easily and efficiently solved by plugging in the answers, which represent the number of girls in Class A. Since the ratio of boys:girls = 3:4, the number of girls must be a multiple of 4. Eliminate B, C and D.

Answer choice E: 12 girls in Class A
Since boys:girls = 3:4, 12 girls in Class A means 9 boys (since 3:4 = 9:12).
Since Class A has 1 more boy than Class B, boys in Class B = 9-1 = 8.
Since Class A has 2 more girls than Class B, girls in Class B = 12-2 = 10.
In Class B, boys:girls = 8:10 = 4:5. Success!

The correct answer is E.

[lunarpower]

agreed with GmatGuruNY -- yes, there are PS problems that give unnecessary information, but they are rare.

presumably, these problems are rare because this whole idea (deciding which information is / isn't necessary in order to solve the problem) is the central theme of DS, which already encompasses half of the math section.
this is also the reason why you hardly ever see PS problems on which the last answer choice is "not enough information given" (such as OG12 #180) -- because that's already the whole purpose of the DS section.

--

incidentally, the upcoming changes to the GMAT (scheduled to be implemented in 2013) tentatively include lots of problems on which most of the information is irrelevant.
one of the major new problem types presents an entire page full of quantitative information and then asks a series of questions, each of which requires you to use only one or two pieces of information from that entire page. it's up to you to figure out which pieces of information are relevant to each question.

the reason for the increased emphasis on this type of problem is that, honestly, it's an essential skill for anyone who wants to work in management.
think about the situations that managers encounter: there are hardly any situations in which the manager is simply handed a complete set of information -- in which all facts are relevant and must be used -- and asked to make a routine decision. instead, almost all important decisions that a manager makes, in general, involve sorting through a great deal of information and deciding what is, and what isn't, important.