How many carbon atoms enter the electron transport chain per glucose molecule?

Zero, the Krebs Cycle converts them all to CO2 which is released.

NO carbons from glucose enter the electron transport chain.The Krebs cycle reactions convert all of them to carbon dioxide, which is released. In Krebs cycle both of the pyruvate molecules get converted into NADPHs and FADHs. In the electron transport chain, each molecule of NADPH produce 3 molecules of ATP and each FADH molecule produce 2 molecules of ATP. In the Glycolysis (consume 2 ATPs and produce 4 ATPs) and Krebs cycle we produce 4 ATPs, 10 NADHs, and 2 FADHs . So at the end we have 38 molecules of ATP . As two were consumed in Glycolysis so we have 36 ATPs net .

29.1-12=17.1the answer is this one

The mass has escaped into a gas which you can not see so the law stays true.