number sense in action

I have it!

Number sense means you can look at an item on SAT-Math and instantly eliminate two of the choices.

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Actually, I have no idea whether being able instantly to eliminate two of the four choices on an SAT math item is or is not an example of number sense.

I do think I-can-eliminate-two-answers is a distinct stage in the novice to experienced to proficient sequence. Last summer, visiting Susan S, I discovered that I could correctly pick the answer to the quadratic function problems in her ACT prep book more often than not. Just a year before I hadn't been able to do any of these problems on a practice SAT test, nor could I eliminate any of the 4 alternatives.

I've seen the same thing with C. at various points. Suddenly he'll be guessing right. He still doesn't know what he's doing, but he knows the answer -- knows it, or has a pretty good idea.