Finding Vo in terms of Vs for an ideal op-amp circuit

[edgarpokemon]

Homework Statement

http://www.chegg.com/homework-help/questions-and-answers/ideal-opamp-find-v0-terms-vs-need-help-solving-vp-firstly-understanding-parts-q8278769

Homework Equations

I am having difficulties with this problem. It asks to find vo in terms of vs. For the gain, I got an equation of Vn/10 + (Vn-vo)/40 = 0, so vo=5Vn. Since the op amp is ideal, vn=vp. Now for Vp is where i am having trouble. I have an equation of [48/60]vs - [48/60]4V=vp. But i am not sure if this right, I am not getting the correct answer. Help!

The Attempt at a Solution

 

[rude man]

1. Label all resistors R1 - R4 and the 5V source V. Don't substitute with numbers until your calculations are done. I can't follow your numbers.
2. Compute vp, then vp=vn as you say, that gives you the current thru the feedback resistor and the output follows immediately.