Great game for kids
This would be a great game for parents to have their kids play in order to practice fast-math. Even adults could use this if they work in retail or another environment where quick addition and subtraction skills are required.
A couple of suggestions. Have the cursor automatically start in the 'text box'. It was a nuisance to have to click into it to start typing my answer.
Perhaps the first wave can be addition, the second subtraction, a combination of the two, the fourth multiplication, the fifth would be slightly advanced multi, then the sixth could be another mixture and so on and so forth.
It would be nice if there was progression in the challenge so that the player is learning. You would even provide 'math tips' or tricks if the player is defeated too often.