What is a math problem that you think covers the most topics in math? Topics could be: logarithms, matrixes, or arithmatic
“is it decidable by a polynomial time algorithm whether two square matrices with non negative integer entries that are shift equivalent are also strong shift equivalent?” maybe not most topics, but many. … it is still an open problem, and considered very hard …lol
Once you have defined the problem the answer will be 42.
Phasor math. It encompasses everything – real, imaginary, log, algebra, matrix, as well as differential equations and laplace transforms.
@daren: I mean one math problem. not site, but thanks for the share anyways. A math problem that uses the most math concepts
The Logic Gate connections of an Arithmetic Logic Unit.
Bayes Theorem. No matter what it is, you need to know how closely it represents reality. 
Firstly, mathematics has no problems, it only offers challenges. 
Secondly, is it possible you’re all thinking too complicated? I think the OP just wants a concrete math example for which to solve you need as many content areas of mathematics as possible, and not some (still unproven) theorem?
A good starting point are engineering topics. They usually have complex systems, physics based, with static or dynamic systems, electricity, finite elements, torsion, torque, you name it.
To describe the systems you usually need to combine a lot of higher math, and to solve the systems you need the rest
Ok, then to phrase it to arexma’s liking, what is a math challenge that you covers the most topics, preferably from K-college level
Economic principles maybe?