How to instantly tell if your answer is impossible

Before even checking your calculation in detail, ask yourself a simple question: does this result make sense in the context of the problem?

A probability of 1.4. A negative length. A 130% success rate. A 200° triangle angle. These are immediate signals that an error slipped in somewhere, before you even recount every line.

This reflex also works in reverse: if a problem asks for the number of students in a class and your answer is 23.7, something's wrong — the context requires a whole number.

Make it a habit to ask "is this number plausible?" before moving to the next question. It's the fastest quality check there is, and it catches a large share of calculation errors before they cost you points.