Solving a problem in physics section 70 point 28 electr

28. A flat air capacitor, charged and then disconnected from the voltage source U1 = 400 V, was connected in parallel with an uncharged capacitor of the same size and shape, with a dielectric between the plates. Determine the dielectric constant ε of this dielectric if, after connecting the capacitors, the potential difference decreased to U = 100 V.