Real life (which I interpret as everyday personal life), business life and entrepreneurial life are very different things.

For real life, I'd recommend the basic mathematics of finance required for managing accounts and budgets, compound interest and amortization calculations etc. This is basically advanced arithmetic as taught in accounting classes. Learn to use Excel and graphing functions well.

For business life in traditional jobs, optimization would be my top pick. Discrete/combinatorial is more useful than continuous. Linear programming is a very general skill that is widely useful. Basic statistics would be my second suggestion. Not actually very useful in solving problems because useful data on which to do statistics is hard to get, but because a lot of communication uses basic statistical arguments. People who actually use advanced stats modeling tend to be specialists you call upon as needed, often not even in-house.

For entrepreneurial life, stats is vaguely useful if you are actually doing an analytics-heavy startup idea, but in most cases it is an expensive luxury beyond basic routine and vanity metrics, for which Google Analytics suffices. You don't need a PhD in stats to deal with basics like acquisition rate graphs and page views or interpreting an A/B test result. In general, learn the most relevant math for your product, since product = company in the early stages. So for social Internet startups, some graph theory might be helpful so you understand things like weak links, small worlds etc. For visual-heavy/graphics-heavy products, geometry might be useful.