Copper is often added to paint for ocean-going ships and boats to discourage the growth of marine organisms on their hulls. Unfortunately, the copper dissolves into the water and is toxic to the marine life in harbors where these boats are docked. When copper(II) ions reach a concentration of 9.0 micrograms/liter, the growth of some marine life slows. How many copper ions are found in a liter of water containing 7.5 micrograms of Cu2 per liter?