Transitive verb and Godan verb
たす
三足す二は五です。
Three plus two is five.
このスープ、もう少し水を足した方がいいんじゃないかな。
Maybe it would be better to add more water to this soup.
ここにもう少し文章を書き足すことはできますか？
Can you add a little more to this sentence here?
Remember 足りる? That is "to be enough." How can you make something be enough? You keep adding things to the pile until you reach that point where there are enough! That's why this verb is to add, because you keep adding until you 足りる. Maybe you add so much that someone tries to sue (す) you for that nasty pile of squirrels you keep outside.
The reading is the same as 足りる, so if you know that you can add things up to this.