Neon Average Atomic M?

Neon has three isotopes: neon-20 (90.5%) neon-21 (0.2%) neon-22 (9.3%). What is the average atomic m and how do I find it? I don't understand! thanks so much.. please don't just give the number I don't understand howww, and please use simple terms, I googled it and it was not simplified a bit..