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Riddle (simple arithmetic problem/illusion)
I don't know if this is off-topic, but I recently saw this riddle, which I believe many of you have seen before:
There are three men travelling. They stop at a motel for the night. They find that one room for one night at that motel costs \$30. So each contributed \$10, and they went to their room. The manager caught the wind of this, and realized that a mistake has been made, as the room actually only costs \$25. He sends a porter with five \$1 notes to repay the guys. Along the way, the porter thinks "No way these guys are going to divide \$5 by 3. I'll help them. " So he takes \$2 out for himself, pays each of the three guys \$1, and leaves. So, originally the three guys have paid \$10 each. Now that they have all received \$1, they have paid in total \$9 for the room each. 9 times 3 makes \$27, plus the \$2 in the porter's pocket, that makes \$29. What happened to the \$1?
Now I know that some of you have heard or seen this riddle before, but what I'm puzzled with is the mathematics behind this riddle. What exactly happened so that there was only \$29? As far as the question goes, it is all quite logical to me, so what exactly went wrong?
Can someone please explain? Thanks!