The controls aren't always responsive, but I enjoyed the puzzles. It's a very simple, intuitive concept but surprisingly deep in terms of variety of puzzle design. Two little things: The numbers are quite hard to read at times, and the "Next" button is rather small given how important it it.
As an after-school math instructor, this is the kind of game I wish teachers assigned instead of addition practice sheets. (Though it would need to be a lower difficulty of course.)