How to solve this proportion
Okay I think I'm just having a major brain block, but I need help solving this proportion for my physics class.
What's confusing me is the solution manual to this problem lists writing the proportion as,
and then proceeds to solve the problem from there... but that doesn't seem right to me. Usually you would cross multiply a proportion and solve, but they seemed to do some illegal math or something. Could you guys work me through how to solve this? This answer is 47 by the way.