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David Walker

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The Stealing Mailman

He didn't have to unlock it to put his own lock on.

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The Stealing Mailman

The given solution is rather roundabout, especially if Kevin and Sophia are going to be exchanging a lot of letters. Here's a more efficient one: 1. Kevin mails Sophie his lock (in an open position). 2. Sophie puts her letter in the box, along with her own lock (open), locks the box with Kevin's lock, and mails it. 3. When Kevin receives the box, he opens it with his key. After reading Sophie's letter, he puts his own letter in the box, along with his (open) lock, and locks it with Sophie's lock. And so on.

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