A capacitor of capacitance 1μF  withstands a maximum voltage of 6 kV, while another capacitor of capacitance 2μF  withstands a maximum voltage of 4kV. If they are connected in series, the combination can withstand a maximum of

Ceff=1×21+2 μF=23μF Capacitor 1 can store a maximum charge 1μF × 6kV = 6mC . Capacitor 2 can store a maximum charge of 2μF × 4kV = 8mC. Since they are in series the charge in each capacitor = 6mc∴        Ceff. Vmax=1μF × 6kV ⇒Vmax=9 kV