How does my book come to this conclusion?

[jean28]

I'm starting my Digital Electronics course and I'm having trouble understanding how this example from my book gets to it's conclusion.

It's about designing an inverter with certain specifications, but how do I find the value of W/L? How do I get the 2 that the answer says?

Here are the pictures of the exercise and another picture with formulas that may or may not be relevant.

Picture of inverter:
http://i1226.photobucket.com/albums/ee410/jean28x/image2_zps9e7f8724.png
Exercise, part 1
http://i1226.photobucket.com/albums/ee410/jean28x/image_zps136b69f7.png

Exercise, part 2 (answers):
http://i1226.photobucket.com/albums/ee410/jean28x/image_1_zps32b9a8ee.png

Relevant equations (?):
http://i1226.photobucket.com/albums/ee410/jean28x/image_2_zpse3d8245e.png


Thank you all very much. I'd just like to understand how the book got to the answers:

W/L = 2
R = 48 kohm
power = 125 microWatts

 

[CWatters]

When the FET is conducting they want V_GS to be 0.1 and I_D to be 50 * 10^-6. Ohms law says R_DS=V_GS/I_D.
Rearrange the equation they give to get W/L. I agree with their answer 2.