Thanks for the feedback! If the hard level is pretty hard, wouldn't adding more operations make it even harder? I kept the number range small because I felt it would be too hard to do mentally otherwise, but I'm definitely open to extending it.
The idea was novel for me but I was quite certain it has been done before in one way or another.
It would probably make the hard level even harder (assuming unique solutions then definitely) but for the lower levels it would provide a wide range of play styles and so more replayability - I'd probably be more inclined to play intermediate with wider ranges and/or other operations than to play hard in "simple" mode.
