What math is essential to the art of rationality?

I have started to put to­gether a sort of cur­ricu­lum for learn­ing the sub­jects that lend them­selves to ra­tio­nal­ity. It in­cludes things like ex­per­i­men­tal method­ol­ogy and cog­ni­tive psy­chol­ogy (ob­vi­ously), along with “sup­port dis­ci­plines” like com­puter sci­ence and eco­nomics. I think (though maybe I’m wrong) that math­e­mat­ics is one of the most im­por­tant things to un­der­stand.

Eliezer said in the sim­ple math of ev­ery­thing:

It seems to me that there’s a sub­stan­tial ad­van­tage in know­ing the drop-dead ba­sic fun­da­men­tal em­bar­rass­ingly sim­ple math­e­mat­ics in as many differ­ent sub­jects as you can man­age. Not, nec­es­sar­ily, the high-falutin’ com­pli­cated damn math that ap­pears in the lat­est jour­nal ar­ti­cles. Not un­less you plan to be­come a pro­fes­sional in the field. But for peo­ple who can read calcu­lus, and some­times just plain alge­bra, the drop-dead ba­sic math­e­mat­ics of a field may not take that long to learn. And it’s likely to change your out­look on life more than the math-free pop­u­lariza­tions or the highly tech­ni­cal math.

I want to have ac­cess to out­look-chang­ing in­sights. So, what math do I need to know? What are the gen­er­ally ap­pli­ca­ble math­e­mat­i­cal prin­ci­ples that are most worth learn­ing? The above quote seems to in­di­cate at least calcu­lus, and ev­ery­one is a fan of Bayesian statis­tics (which I know lit­tle about).

Se­con­dar­ily, what are some of the most im­por­tant of that “drop-dead ba­sic fun­da­men­tal em­bar­rass­ingly sim­ple math­e­mat­ics” from differ­ent fields? What fields are math­e­mat­i­cally based, other than physics and evolu­tion­ary biol­ogy, and eco­nomics?

What is the most im­por­tant math for an ed­u­cated per­son to be fa­mil­iar with?

As some­one who took an hon­ors calcu­lus class in high school, liked it, and did alright in the class, but who has prob­a­bly for­got­ten most of it by now and needs to re­learn it, how should I go about learn­ing that math?