Is this a Run on sentence?

Multiplying fraction and decimals isn't that hard, I can show you how.

No. It is a comma splice - because there is a comma that is incorrectly connecting two independent clauses. In addition to the comma, you must add a coordinating conjunction, such as 'and' in this example.

Of course, you could also end the sentence instead by adding a semi-colon or a period to create two separate independent clauses.

A true run-on sentence is one where two sentences run together without any punctuation.