Here are some strategies that NOVA suggested on guessing. Personally I don't know how valid it is, but if you're really short on time following the strategy can give you a solid guess.
1. on hard problems, if you're ask to find the least (or the greatest) number, eliminate the least (or the greatest) choice
2. on hard problems, eliminate the anser choice "not enough information"
3. on hard problems, eliminate choices that are merely repeat numbers from the problem
4. on hard problems, eliminate answer choices that can be derived from elementary operation
5. after elimination, choose the more complicated or more unusual answer choices
what do you think guys?