This is one question which bothers most CAT aspirants - "Can I guess?"
"If you can eliminate all options except two, guess..." says Gaurav Bhattacharya, a 100 Percentiler in CAT 2007. Well, I have to dis-agree with him here, as guessing is something much more difficult than actually solving the question.
Why do I say this?
Well, the IIM professors who make the CAT, design it so as to eliminate all chances of people making the cut just-by-chance. They are looking for real intellect and not beginner's luck.
Guessing is something which can be done only if you have had already experienced similar questions many times before and are sure about what the answer could look/be like. For a problem/question which appears completely new to you, it is wise to let it be and use the time to solve another problem which you know how to solve.
What about guessing in verbal section?
Most students believe that the verbal section is pure luck and works on pure guessing. I know many 99+ percentilers who believe that the law of averages will ensure a 50% accuracy if they answer most of the questions. However, this is not true. Though the verbal ability section looks ambiguous and the answers in the key look unlikely at the first glance, a thorough analysis of the answers will prove otherwise. One needn't be a Charles Dickens to crack the CAT verbal, nor does he need to answer all the 40 questions.
A person answering 20 questions with 5 mistakes would obtain 55 marks (ie., 75% accuracy) which would be a 96+ percentile - enough to fetch him all calls. While a person answering 38 questions with 19 mistakes (50% accuracy) would be getting the same 57 marks. However, answering 20 questions would take much less time and effort compared to answering 38 questions. A person answering 20 questions can even leave out a whole Reading Comprehension passage which he finds difficult or time consuming while a person answering 38 questions would have spent at least 10 - 15 mins on the same passage and also guessing through it to attain such a high attempt rate.
A similar logic can be applied to QA or DI sections.
Solving 12 problems with no mistakes out of 30 in QA/DI would fetch you 48 marks which is 98+%ile while answering 18-20 questions out of 30 with 6-8 mistakes can cost you a call from the top 3 IIMs.
Its not the number of questions you answer, but the number of questions you answer RIGHT.
Remember, you don't need to top in the CAT - you just need to clear all cut offs and clear the final cut off - for which you don't need to be a super human.