The captain, in order to save his life would give the only coin to the next senior pirate (the potential next captain). Seeing this, the rest of the 4 members would plan to kill the pirate. Once the member is dead, the captain would then decide to hand over the coin to the next senior pirate, who also gets killed and it goes on until only 2 members are left. At this point, the junior most member wouldn’t dare kill the captain alone and hence, the captain not only saves his life, kills the other pirates but also gets the coin.