足りる and 足す: the amount needed and the amount added
Distinguish having enough from adding a shortfall, and track the required total, present amount, and addition in examples.
Whether there is enough
椅子は十脚で足りますか。 asks whether ten chairs are enough. 足りる, read たりる, evaluates an amount against what is needed.
人数が増えたので、椅子が足りません。 says there are not enough chairs because the number of people has increased. It means fewer than needed, not necessarily no chairs at all.
Add the missing amount
椅子を二脚足してください。 asks someone to add two chairs. 足す, read たす, takes を for the added items and has the te-form 足して.
今八脚あって、十脚必要なら、二脚足せば足ります。 means that with eight chairs present and ten needed, adding two will be enough. Distinguish the two added chairs from the total of ten.
Apply the distinction to flavor and time
味が薄いので、塩を少し足しました。 says that someone added a little salt because the flavor was weak. 足す describes adding an ingredient, not only numerical addition.
あと十分あれば足ります。 means that another ten minutes would be enough. あと indicates additional time here, not a total task duration of ten minutes.
With eight chairs available and ten needed, how many must be added to make the number sufficient?
Show the answer and why
Add two. 二脚足す is the action of adding them; the resulting total of ten gives 十脚で足りる.