It is a bit harder to write your own good quantitative question stems (trust me, I once wrote a big chunk of a test bank for my old university’s quantitative methods subject — it took forever!) but there is still some useful thinking like a test writer to do for your normal multiple-choice problem solving questions on the GMAT, GRE or SAT. In particular, if you are a test writer, you want various percentages of people to pick the wrong answer over the right answer. So how do you get them to do that?
Well, harder problems are harder (generally) because they require more steps. Many non-TestSmashing test takers will forget where they are heading on a question and just kind of stop when they feel they have taken enough steps — or stop when they have a number that matches an answer choice. So how do you gull these poor souls into choosing a wrong answer? Make sure that each step’s answer appears in the answer choices! You, of course, are going to remember what the test really tests, and make sure you actually answer the question being asked, right?
And, while it breaks my heart to say it, waaaay too many people who feel helpless on a question will just randomly combine the numbers in a question. (Are you one of those people? For the love of God and all that is good, resolve to STOP TODAY and never, ever do that again!) Fill in the remaining wrong answer slots with such random combinations.
If you want active practice thinking like a test writer for problem solving questions, pick up an algebra text book (or search the internet) and find some open-ended word problems. Solve them, and note along the way the intermediate steps. Come up with 5 answer choices. Post in comments if you want!