Equivalent Liquid Perm: 2017 #7

A gas filled rock core has an average flowing pressure of 1.67 atm with a corresponding gas permeability of 35 md. When the mean flowing pressure is raised to 2.5 atm the gas permeability is 40 30 md. This core’s equivalent liquid permeability is nearest to:

This problem is just like the GB example. We find mx + b then project to the x axis:
1) m = (30 - 35)/(1/2.5 - 1/1.67)
    m = -5/(0.4 - 0.6)
    -m = -5/0.2 = -25 (this is the slope).
2) -25(0.6) = -15.
3) -15 + 35 =  20 or (D).

The inverse slope makes the math awkward, so I recommend doing a quick graph sketch (not on graph paper, you won't have any). On this problem the graph is identical to the Guidebook's so it's easy. If you had a hard time with this one, redo it over and over until it makes intuitive sense. For more examples check out Bradley, problems online, or the 2016 Practice Exam. Fair warning: many smart guys unexpectedly miss this problem type under time pressure. Don't trust your math; graph it out and ensure the answer "makes sense".