Gaby, a good mathematician, thinks of a whole number between 1 and 100 inclusive. What is the least number of questions Jack needs to ask her, and what questions should those be, if he is to work out with absolute certainty Gaby's number. Gaby will answer Jack's questions truthfully, honestly, and to the best of her knowledge, and is only allowed to respond "Yes", "No", or "I don't know".
There is a difference with the apparent duplicate: Gaby has the option to honestly answer "I don't know". It makes a big difference! Puzzle inspired in another elsewhere in this site.