1
to add (numbers)
６０にするためには１７に何を足せばよいの。
What do you have to add to 17 to get 60?
2
to add (something);to top up (with something)
もしコーヒーが濃すぎれば、いくらかお湯を足して下さい。
If the coffee is too strong, add some more water.
3
to take care of (e.g. one's business)
See also:
用を足す (ようをたす)