If there was one mathematics subject I'd tell people to pay close attention to, and at least master the basics in, it would be probability.
Not only will it help you stay away from gambling at casinos, cause you'll know exactly WHY yer gonna lose, and how fast....but it really helps in just about any decision you'll ever make.
"Yeah, maybe it's possible that monkeys might fly out of my ass someday, but it ain't the way to bet." Unless you're Jim Carrey. And I STILL think that was done with mirrors.
Looking at most things in life as a "bell curve", really helps in planning for disasters. Keeps you realistic. Defeats the tendency towards an "all or nothing" personality. Which keeps you even-tempered.
Life and people don't surprise you disastrously as much, 'cause you had a sense of the odds going in. That sorta thing. It also explains exactly why the "80-20" rule is so important in being effective in your time management, for instance. Perfectionism kills.
It also teaches you that being 10% to 20% better at your job than your co-workers is good enough, and to devote the rest of your time to your family. (Although it's better that only your boss and HIS boss know this, rather than your co-workers, for social reasons.
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I really can't stress enough how much the study of probabilty has exceeded ANY other branch of math in being of use to me. If they didn't teach it to you in high school, grab an introductory primer on it and read it. One of the best investments you'll ever make.