I can’t do addition.
Can I participate in the addition contest as a spectator?
Today we studied addition in school.
Remember how 足す means "to add?" Add calculate to that and you'll have addition! It's the type of calculation you're doing when you add.
This is the kun'yomi reading for 足 and the on'yomi for 算. And the さん reading is rendaku'd into ざん. This is the same for all the math-ish words (addition, subtraction, multiplication, and division), so try to remember that, because you'll see it again!