yep simples. this is essentially how encrypted email started.
The only annoyance is that the box(B) gets sent three times b4 Kevin can read the message. [B+Slock; B+Slock+Klock; B+Klock]

I agree, the answer is not valid as it involves exchanging locks & keys.

I’m guess that ‘The pair cannot interchange keys/locks.’ means that one’s key cannot open the other’s lock – poorly worded i think…

double locked box

Close one lock to the lid, then place the other through that lock and lock it to the box. Each can unlock their own lock from the other.

Alternate (more efficient) answer: They use a box that can be opened if either of 2 locks are opened, the closed lock functioning as a hinge.

“2) Kevin adds his lock to the box and mails it back to Sophia.”
Technically they didnt INTERCHANGE locks, but she definitely sent him her lock. Lame.

Why don’t they just mail their empty unlocked boxes first, then have the other put their letter in, lock it and send it back. Much easier ;)

Better answer: Sophia puts her unlocked lock in the box and mails it to Kevin. Kevin puts his letter and his unlocked lock in the box, locks it with Sophia’s lock, and mails it back. Etc.