A parallel-plate capacitor using a dielectric material having an r of 2.5 has a plate spacing of

A parallel-plate capacitor using a dielectric material having an εr of 2.5 has a plate spacing of 1 mm (0.04 in.). If another material having a dielectric constant of 4.0 is used and the capacitance is to be unchanged, what must be the new spacing between the plates?