Scott Flansburg is best known for being the Guinness Record holder as a “human calculator,” but the most amazing part of his ability is that, literally, anyone can attain this “superpower” as well. His methods are incredibly straightforward and can be learnt for free!
Feel free to watch these videos at your own leisure, and then surprise friends at your sudden math prowess!
Aside from those videos, there is the website “Math Is Fun” which covers math from counting all the way to calculus in a fairly straightforward manner. There is also “Khan Academy,” which began as a series of videos but has since become a site of its own.