The sum of two numbers is - 12. One of the numbers is 4. What is the other number?

This could be done in two ways. If the first number is 4, then you could add 4 with - 16. 4 + (-16) is - 12. But it could also be - 16 + 4, which still equals to - 12.

But in the end, your answer is the same : - 16 is the other number.