What we came up with looks something like this:

private object FizzBuzz(int value)
{
  if (value % 15 == 0)
    return "FizzBuzz";

  if (value % 5 == 0)
    return "Buzz";

  if (value % 3 == 0)
    return "Fizz";

  return value;
}

Don't worry if you chose to solve the problem a different way. The important thing is that you gained knowledge and understanding during this exercise. Additionally, you now have a comprehensive set of tests and you're comfortable refactoring and/or adding functionality.