Ca = C5 parallel with C6 = C5 + C6 = 6 μF + 6 μF = 12 μF Cb = C4 series with Ca = C4*Ca / (C4+Ca) = 12*12/12+12 = 6 μF Cc = Cb parallel with C3 = Cb + C3 = 6+6 = 12 μF C = C1 series with C2 and Cc , C1=C2=Cc = 12 μF Ec = E1 = E2 = 45V/3 = 15V Eb = Ec = 15V Qb = Cb*Eb = 6* 15 = 90 μC Q4 = Qb = 90 μC E4 = Q4/C4 = 90/12 = 7.5V Voltage on C4 = 7.5V