I seen this kind of gameplay before. (Brain Age, etc.) Also, the hard mode repeated questions the first time I tried it. Nevertheless, this definitely engaged my brain. In fact, there was a moment where I was like having different numbers at the same time to do the math, and I still managed to answer the question even though it felt like I shouldn't have been able to do that. This game would be immensely improved by a leaderboard (even a offline one though online would be superior). Also, this is more of a personal preference, but a mode for stuff relating to the powers of 2's would maybe be satisfying to me since because I dealt with those kinds of numbers before, it could potentially be like having a superpower where here is this hard question and I solve it fairly to very easily and quickly.