Doesn’t actually use math to check the answers!
My 6 year old playing on the easiest settings pointed out that correctly filling out the puzzle does not always cause the app to accept the solution. For example, I checked and saw that while 3+4=7, and 4+3=7 the app only will accept one of these solutions! If you don’t get the one it wants the first try, it seems to be smart enough to recognize that switching the 3 and 4 doesn’t change the solution and so leads to a condition where it becomes impossible to finish the puzzle! I tested at the highest available level (that should be appropriate for my (13 year old) and found that some puzzles had up to 8 correct solutions but again, only one opportunity to get the one solution the app expects.