First and foremost, please remove the last statement from your post as it talks about a specific exam question. This is not allowed per the NCEES rules.
I don't get what you mean by "simple way"? There is a formula in HCM Chapter 16. I don't think that there are other ways to determine it, unless you have access to some software packages, but that is beyond the scope of the examination. Just follow what is written in the HCM.
I still can't figure out your question, "easy way to calculate with this much traffic at intersection, bla bla". Of course, the green time depends on the volume at the intersection, as far as my limited knowledge goes there is no magic number to found here.