I second this recommendation. It's a lovely way to explore some of the beauty of mathematics. I have a BA in math and I think the most-useful mathematics for e

I second this recommendation. It's a lovely way to explore some of the beauty of mathematics. I have a BA in math and I think the most-useful mathematics for everyone is probability (math theory and basic calculations). This is because you'll be better able to deal with (quantify) uncertainty. The concept to aim for is (learn enough so you can understand and use) expected value calculations - which are, I think, the foundation of rational decision making under uncertainty.