This person is vertically challenged and cannot reach any of the buttons above the twentieth? :-D
This had me stumped when I first heard it. It actually is fun to see the same thought patterns for many people as they try to figure it out. But the answer is so basic.
I have used this for young children, and many times they will solve it much sooner than adults. I think it's due to them visualizing what happens in the elevator, and their heads are not filled with so many side distraction like asking about girlfriends, evil intentions, etc. They just solve it on basic practical matters.
I once had a group so stumped, I had to draw a picture of the elevator buttons, and asked them to explain what would happen if he was going down or up. And it took them just as long to finally figure out that there was more than pushing two buttons. Like Charades, I had to actually go through the motions, before a light bulb had gone off.