Mathematicians sometimes separate good bets from bad ones by using a concept called expected value. Consider the example of betting on the outcome of a die roll. It costs $1 to pick a number between one and six. If you guess the roll correctly, you win $1, and if you guess incorrectly, you lose your dollar. Would you take that bet? It seems unfair because you stand to win exactly as much as you stand to lose ($1), but you’re much more likely to lose (five out of six rolls lose).