Interesting concept and very nicely made! I liked the music, the sounds and the UI. However, I was mostly looking at my keyboard to type the correct numbers, so I wouldn't really see what's going on and what the health bars were at; I just focussed on solving the math as quickly as I could. Perhaps if the intervals between casts was longer I'd have time for that, or if you get 6 possible answers and you have to quickly (with a timer?) select the correct one; then you can stay looking at the screen.