Potential Change in a Circuit Diagram

[daimoku]

[SOLVED] Potential Change in a Circuit Diagram

Homework Statement

http://personalpages.tds.net/~locowise/test/28_29.gif

If the potential at point P is 400 V, what is the potential at point Q?

Homework Equations

Ohm's Law: deltaV=iR
Probably the fact that the sum of changes in potential in a loop of any circuit must be zero.

The Attempt at a Solution

Not real sure on this problem. I tried this:
150V - 2.0i - 3.0i - 50V = 0V and found i to be 20A.

I'm kind of puzzled and can't seem to relate the material in the textbook to this problem. Where does the 400V potential come into play? Thanks for your help!

 

[daimoku]

I'm still not really sure this is the proper way to approach the problem but apparently the solution is correct. Here's what I did:

Used 20A from above.

400V - 50V - 3.0ohms * 20A = 290V