About the no of trip


I could not understand the logic.Somebody please explain…


Alice places the Medicine inside the chest and locks with LockA (keeps his KeyA with himself).
Eve transports (Trip 1) the Medicine to Bob. But Bob cannot open it.
So Bob locks the chest with his LockB.
Please note that at this state, the chest has both the locks. ie. LockA as well as LockB.
Eve transports (Trip 2) back to Alice with both the locks.
Now, Alice unlocks his LockA, and keeps it with himself. But still, the LockB is on the chest (so Eve cannot steal the medicine).
Eve transports (Trip 3) to Bob with only LockB.
Bob unlocks his LockB and gets the medicine.