Taxation Law Nightmare: Section 68 Exemptions

Main problem hai, guys! I'm stuck on section 68 exemptions and I have 3 days left for my exam. I've studied the entire Syllabus but still, this one thing is killing me. How can we determine if an expense is incurred wholly and exclusively for the purpose of business? What about when we have partial business use? I've been reading the judgment in CIT vs. Kelvinator, but still, my brain is not getting it. I've made flashcards, notes, and everything, but when it comes to applying these exemptions in a scenario, I freeze. Does anyone have a clear understanding of this? Please help!