Try re-drawing the circuit like so:
simulate this circuit – Schematic created using CircuitLab
(It is much easier to discuss a circuit when you use reference designators (R1, R2...) on the components.)
You correctly determined that R3 and R4 are in series, and are equivalent to a 60K resistor. You also found that R3 and R4 in series, are in parallel with R2, to make an equivalent 20K resistor. You then added that combination to R1, and got lost.
R1 in series with the combination of the other three resistors make a voltage divider, giving a voltage (to be determined by the student) at V1.
R3 and R4 make another voltage divider, multiplying the voltage at V1 by 1/3, giving the desired voltage at Vo.
You can easily solve this problem by looking at the resistor values - there's no need to calculate currents (although doing such calculations may be useful practice...)