Okay, here's something interesting...
It only works with subtraction!
Say you add them together. There is NOT an infinite number of positive numbers whose sum is a given number. If you allow negative, then this might work, but you could just consider that to be subtraction anyway.
If you multiply them, then all you have to do to crack it is factor the ciphertext and try all of the different combinations of factors.
If you divide, then theoretically there is an infinite number of possibilities, but you would lose precision by writing it as a decimal and writing it as a fraction would be useless.
The only one that works correctly is subtraction. This makes it unbreakable even though it's extremely simple.