Stop! If you haven't taken a look at the Quantitative Comparison Questions for the revised GRE, then head there, answer the questions, and come back. This page gives you the Quantitative Comparison answers and it will make no sense to you if you haven't read the questions, first.
The Answer to Quantitative Comparison Question 1
In this question, the answer has to be D, the relationship cannot be determined from the information given, and not just because there were no numbers involved. You could potentially still figure out who had the most cash. However, because the relationship between Sam and Keisha is never explained, we don't know if Sam has more or less money than Keisha. Therefore, you cannot determine which quantity is greater.
The Answer to Quantitative Comparison Question 2
To determine the difference between the numbers, we have to make x as large as possible, or slightly less than 8, and make y as small as possible, just less than 3. The answer to x-y, then will have to be a little less than 5, so the answer is B.
The Answer to Quantitative Comparison Question 3
If this is tough for you, then draw it out. You'll be able to see that the distance to the origin in both cases is equal, or answer choice C. The segments joining points M and N to the origin are both hypotenuses of right triangles with sides 3 and 7. See? Easy cheesy.