An electrostatic experiment uses an air-filled parallel-plate capacitor with plate separation $d$. The capacitor is fully charged using a power supply until the electric field strength between the plates is $E$. The capacitor is then completely disconnected and isolated from the power supply.

The plate separation is then increased to $3d$, and a sheet of nylon with a relative permittivity (dielectric constant) of ${\epsilon }_r=5.0$ is inserted, completely filling the volume between the plates.

What is the new electric field strength between the plates in terms of $E$?