Quantitative comparison gives two quantities and asks which is greater, or whether they are equal, or whether it cannot be determined. The four answers never change, which makes this the most strategy-rewarding item type on the GRE.
Do not solve. Compare. You rarely need a value, only a relationship. If both columns contain the same term, cancel it. If both are positive, you may multiply or divide both sides by the same positive number. Simplify until the comparison is obvious.
Test the awkward numbers. When variables are involved, the students who go wrong test 2 and 3. Test 0, 1, a fraction between 0 and 1, and a negative — those four break most assumptions. A fraction between 0 and 1 gets smaller when squared, and a negative reverses an inequality when you multiply by it.
"Cannot be determined" is a real answer and it is under-chosen. If two different legal test values give two different relationships, you are finished — the answer is D, and there is nothing more to compute. Students distrust it because it feels like giving up. It is not: it is the correct answer roughly a quarter of the time by construction.
One structural fact worth knowing. If both quantities are plain numbers with no variable, the answer cannot be "cannot be determined" — the relationship is fixed, so D is out before you start, which turns a four-way guess into a three-way one.
Time. About 90 seconds each. If you are grinding through arithmetic, you have missed the comparison the question was built around — flag it, answer it, and come back with the section's remaining time.