PE Practice Solution Inquiry

I am currently studying for my PE in April. I have been doing the HVAC and Refrigeration practice problems from Engineering Pro Guides. One of the solutions to a problem uses an equation/conversion that differs from the formula I am familiar with and also contradicts a previous solution with the correct formula. Am I missing an additional step or is this solution incorrect? Solution 15 is what I believe to be incorrect, while Solution 3 seems to use the formula correctly. Any thoughts are appreciated. Thanks.

I am no mechanical eng but I am as well studying for PE. 

I used a total of 3 different references + 1 NCEES practice. I often find foreign equations out of nowhere in the solution that are nowhere to be found in the NCEES official ref. manual.

There's even lots of mistakes with calculation, notation, units and nomenclature confusions in the author-given solutions to their own problems XD

Do not waste too much time on details like those, just learn the topic covered extensively from your old textbooks or online sources then move on. You have far much more to worry about than just one problem.