Three women go into a hotel and ask for a room to share. The receptionist tells them it will cost 30,000 yen, so the women pay 10,000 yen each.
After the women have gone to their room, the receptionist suddenly remembers that the hotel has a winter special on right now, so the room should cost only 25,000 yen. He calls a bell-hop, gives him 5000 yen, and tells him to bring it to the customers.
However, on the way upstairs the bell-hop sneakily decides to pocket 2000 yen for himself. When he gets to the customers' room he gives them only 3000 yen. So each woman ends up getting 1000 yen back.
Let's think about this. Each customer originally paid 10,000 yen and got 1000 back, so they each paid 9000 yen. Additionally, the bell-hop took 2000 yen for himself.
9000 x 3 = 27,000.
27,000 + 2000 = 29,000.
But the three women originally paid 30,000 yen! Where did the extra 1000 yen go?
(This was a riddle from my Japanese class, which had nothing to do with Japanese and everything to do with our teacher wanting us to figure out the answer for her.)