When a solid dielectric was introduced into the space between the plates of an air capacitor disconnected

When a solid dielectric was introduced into the space between the plates of an air capacitor disconnected from the current source, the voltage across the capacitor decreased from 400 to 50 V. What is the dielectric constant of the dielectric?

U1 = 400 V.
U2 = 50 V.
ε0 = 8.85 * 10 ^ -12 F / m.
ε -?
For a capacitor, the following formulas are valid: C = q / U, C = ε * εo * S / d, where C is the capacitance of the capacitor, q is the charge on the capacitor plates, U is the voltage on the plates, ε is the dielectric constant of the dielectric, S is the area of the plates capacitor, d is the distance between the capacitor plates.
q / U = ε * εo * S / d.
U = q * d / ε * εo * S.
Dielectric constant of air ε = 1.
U1 = q * d / εo * S.
U2 = q * d / ε * εo * S.
U2 = U1 / ε.
ε = U1 / U2.
ε = 400V / 50V = 8.
Answer: dielectric constant ε = 8.