Consider the set of northwestern states or provinces ( Montana, Washington, Idaho, Oregon ,Alaska, British?
Columbis, Alberta). If a person chooses one element, show that in three yes or no questions, we can determine the element.
There are seven "elements" in the set.
The first question can help you choose between a subgroup of three or four. In the worst case, you will be left with a subgroup of 4 after the first question. One more question can isolate two, and a third question can isolate one of those two.