Decent idea
This game could really use difficulty settings though. There's a difference between what's challenging and what's impossible. It kind of detracts from the game when you are literally unable to get to the correct answer in the allotted time.
It might be more playable if on the normal mode you moved a bit faster, had fewer knives and maybe added some powerups (health,shield, speed, etc.) and then the harder mode was more like the current version where the game was pretty unforgiving.
Overall though the game was pretty good.