A parallel-plate capacitor is made of two flat metalplates pressed against a thin slab of dielectric material. The capacitor is connected to a power supply, and a potential difference of 90 V is applied to the plates. With the power supply disconnected, the dielectric material is removed and the potential difference between the plates is measured to be 600 V. What is the dielectric constant of the material that was initially used to fill the gap between the plates?