A salvage ship is able to raise a container ?lled with unknown material from the ocean ?oor to the water surface, but cannot raise it above the water. The ship captain knows that the overall density of the container is 3 times the density of water, and he wonders whether one other ship with a crane of equal capacity will be enough to help him lift the container above the water, or whether he will need the help of more than one additional ship.
How many ships with equal lifting capacity will be required to lift the container?