Do you want to learn to do statistical analysis with a tool like R and interpret data? Do you want to learn mathematical axioms and theorems about probability and statistics and how to prove them.

Ideally, I'd like to learn both.

Depending on who you ask it's not. sum/n is the arithmetic mean. There are also other mean's like the geometric mean and the harmonic mean. Depending on the context different mean's can be used.

I know about different means. And I know that sometimes mean isn't appropriate at all (bimodal...) The context is one of the things I'd like to understand.

When it comes to learning notation. Anki is quite good.

Just used it to learn the Greek alphabet. :-)