Boyish blue and girly red?
There were two siblings - Alfred and Brenda. One of them likes red while the other likes blue. Also one of them always tells the truth the other always lies. Unfortunately you don't know who is the truth teller and who is the liar.
Imagine you want to know who likes which color. You are allowed to ask Alfred and/or Brenda for the answer. What is the fewest number of questions you need to ask?
The answer is 1.
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1 solution
You can ask Alfred, "If I were to ask your sister what is her favourite colour, what will she say?" or you can ask Brenda, "If I were to ask your brother what is his favourite colour, what will he say?"
Assume you happen to ask the truth teller what the liar says is his/her favourite colour. The liar will mention the colour he/she does not like and the truth teller will just tell it to you as it is. On the other hand if you ask the liar what the truth teller says is his/her favourite colour, the truth teller will mention his/her genuine favourite colour but the liar will tell you the colour the truth teller dislikes.
Either way you are bound to get a lie. The colour claimed to be the other person's favourite colour is actually liked by the very person you are asking because there are only two people. The other person will simply like the colour other than the one told to you since there are also only two colours.