This is exactly the same as the whiteboarding algorithms questions. They all have a simple answer which is obvious if you have been recently cramming for your CS finals. But you are unlikely to remember them in sufficient detail if you are over a certain age, because no real actual working programmer needs to know that stuff by heart, it's either been abstracted into libraries or you just reach for a book when you need one. A perfectly plausible way to do ageism, right under the noses of any regulators.