Imagine you are on your way to San Francisco. Suddenly the road splits into two with one leading to the right and one to the left. At the fork there is a house where two brothers live – Mike and Frank one of whom always lies and the other one always tells the truth.
Both of them know which of the two roads lead to San Francisco but you don’t. You can ask only one single question and you can ask only one of both brothers. What question do you ask in order to definitely find out which of the two roads leads to San Francisco?
How about “If I asked your brother which road leads to San Francisco, what would he say?” Then take the opposite.
So if the 280 leads to San Francisco and the 880 leads to Oakland, where of course you don’t want to go
Then, if you are asking the truth teller, he will say “My brother always lies, so he would tell you to take the 880.”
If you are asking the liar, he will think, “My brother would say to take the 280, but since I always lie, I will say he would say to take the 880.”, so then he will say, “My brother would say to take the 880.”
