Not as punishing as Maths Blaster. I love the sprite work. Are they your own works?
My game is not so punitive because my sons are 5 and 7yrs old and i want to do game more motivating than punitive. But i plan new mode "time attack", i hope it set the game more difficult for older players.
About sprites, not all are mine, all monsters i taken from public domain opengameart.org.