This is not true. For example you know that the result of 5.123.443 x 9.999.999 can't be 42. But you dont immediately know the correct answer, which is harder.
Making the answer hard to deduce/calculate is only one side of the difficulty. The other side is making the question hard to decipher. I don't think the SAT has any questions that are as straightforward as your example, but I could be wrong.