Laughably awful
This purports to be an educational app, helping one to learn arithmetic. But check this out, copied verbatim:
Subtraction 9
To subtract 8 from any number, you need to subtract 10 from it and add 2. If you normally think differently, then for best results, you need to get used to this new way.
Example:
15 - 9 = 15 - 10 + 1 = 6
WHAAAAA?!!
Delete, bye bye.