physics-current electricity, circuit

[lydigihome]

1. The problem statement, all variables and given/known data

http://img208.imageshack.us/img208/4577/physicso.jpg

2. Relevant equations



3. The attempt at a solution

dont know how to do..
help please, how can i find that is 4V?

[lydigihome]

oh, some small amendment:
that 15ohm at the top righthand corner should be 10ohm

[gneill]

oh, some small amendment:
that 15ohm at the top righthand corner should be 10ohm

But the resistor in the top right-hand corner is 20 ohms...