This won't be on the MCAT, it is 36 in people and 38 in prokaryotes. The reason for the discrepancy? For us respiration occurs in the mitochondria, prokaryotes don't have a mitochondria. 2 of the ATP is from Glycolysis, 6 of the ATP is an inbetween step -the decarboxylation of pyruvate into acetate. 24 is from the Krebs cycle. So we have between 4 and 6 that come from somewhere else. The answer is from the reduction of NADox that occurs in step 5 of glycolysis. In prokaryotes, since respiration occurs in the cytosol, the NADH can go directly to the first acceptor in the electron transport chain in the plasma membrane, a process that we say makes 3 ATP per NADH. In Eukaryotes the NADH must get into the mitochondria to access the electron transport chain, it does this through the glycerol-3-phosphate shuttle. NADH reduces dihydroxyacetone phosphate (thus regenerating NADox for step 5!) into glycerol-3-phosphate. Glycerol-3-phosphate is then transported into the mitochondria where it can reduce a flavoprotien in complex II, we say this process gives us 2 ATP per glycerol-3-phosphate. The glycerol-3-phosphate is converted back into dihydroxyphosphate by this oxidation and it diffuses back into the cytosol. So we see that you get 2 ATP per NADH (x2 for one glucose) produced in the cytosol for eukaryotes and 3 ATP per NADH (x2 for one glucose) produced in the cytosol for prokaryotes.
The 1.5/2.5 thing is supposed to be more accurate, but isn't in every textbook yet. Really it doesn't matter, the ATP doesn't come from the NADH or FADH, it doesn't even come from the electrons, it's strictly from the diffusion of protons through ATP synthase, energy this is dependent on the proton gradient.
