Hint from Hitequest:
Q1=C1U1; Q2=C2U2
U2=C1U/(C1+C2)=4v
from Hector M.:
there is an error on the question with
the 2 capacitors in series (20uf and 30uf)....
The solution is 6 volts instead of 4 volts. The voltage
divider formula is:
V=(C2.Vcc)/(C1+C2)