I have one of the three numbers: 1, 2, or 3 in my mind. I speak only truth. You can ask me just one question for which I will only reply in yes or no or don't know. What question will you ask from me so that you are able to know the number?
What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?