Excellent game that provides a very engaging experience. I particularly liked how the text frequently provided hints on what to do next, so there was never a time where I felt clueless. My only gripe is that the text is quite small and can be hard to read sometimes (at least for me), so it would be great if there was an option to increase the font size.
Why can't we have all missions available at once? Replaying levels just to complete objectives that you have done many times before seems unnecessarily grindy and timewasting, especially when you are already maxed out and have nothing to gain in terms of xp or money. It's a shame that the better perks you need for endless mode can only be unlocked if you complete more missions i.e more grinding.
Showing the possible combinations for each box makes the whole idea of the mathematical operators pointless. You can just view the combinations and play it like regular sudoku without having to do any math.