Imagine someone taking Free-Throw shots in basketball. This is not unlike a series of True/False questions.

Now imagine two candidates or "shooters": one knows 50% of the material. The other knows 80%. Just 20 questions is suprisingly efficient at separating the Pretenders from the Contenders. Only 4% of unqualified candidates can pass a test that has a 75% passing grade. Meanwhile you're certain that you've kept 80% of the qualified candidates with that same 75% passing grade.