Why do we have to handle double's and char's? Do you think that copy-pasting the same code 2-3 times makes the kata better?

All other data types do not matter in this problem, and should return "Invalid Types."

The result of an invalid operation is indistinguishable from normal methods' behavior.

Ops.Operate.Sub("Hello World!", "orl") == "he wd!"; // String

This example says that the result should be in lowercase which is wrong.

Solutions for dice with fixed 6 faces can be hardcoded. What about making the number of faces a parameter?

Inconsistent naming of variables and description:

(X^n)*(e^(-x))/n!

The formula contains both X, x and n, however the description talks about X and N. Apparently, X and x are the same, and N is n. Also, n suddenly becomes 'result' and x 'mean' in the C# code. This is not stated in the description. Albeit trivial, I would add it.

no random tests

sample tests are broken

tests/Fixture.cs(6,17): error CS0542: 'Tests': member names cannot be the same as their enclosing type