Why focusing on the right answer, is the wrong answer

We’ve all been there,  someone asks you a question you’re supposed to know, and your mind goes blank. This probably happened more when you were a child, but even as adults we can suffer from mental brain freeze. So what would 20% of that be? How much do we each have to pay if the total bill is X? What number do we times 2 by to get Y? Perhaps if it was a written question, where we were in the safety of our own heads, we’d work it out comfortably. So what actually happens to us, what happens to students when they are put under pressure?