Basic probability (conditional; Bayes theorem; correlation; expected value of order statistics; IID sequences), statistics (linear regression, think how beta varies under changes in the data), brainteasers (inference for ordered information; e.g. five people arrive to a place, incomplete information on arrival times and order, which option is necessarily true (or false)), speed of algorithms and data structure data access (sort, indexing; how fast does a structure access data), linear algebra (matrices, matrix operations; determinants and their properties), zeroes or maximums of curves under domain constraints (also in higher dimensions), mathematical series (where does a given Taylor series converge).
