Basic algebra states that unless there are factors on both sides of the equal sign, you can't find the variable...in this case the variable is the user input, which could be absolutely anything, depending on who the person is that is providing the input. And since the variable could be anything, the answer is also quite possibly anything.

Simply put, in the current state of the equation, the problem is quite unsolvable.

However, since the answer to the equation is derived from the addition of user input, the user can and likely will choose what answer he or she desires, thus controlling the outcome.