For cis-1,3-dimethylcyclohexane take: a CH${}_3$/H syn-diaxial interaction $=3.7\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ and a CH${}_3$/CH${}_3$ syn-diaxial interaction $=15.4\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. What is the energy of the diaxial chair relative to the diequatorial chair? (Count each distinct interaction once.)

In the diaxial chair the C1 methyl interacts with the axial substituents on C3 and C5, and the C3 methyl with those on C1 and C5. The CH${}_3$(1)/CH${}_3$(3) interaction is a single shared interaction (15.4), and each methyl also faces the axial H on C5 (2 $\times$ 3.7 = 7.4). Total $=15.4+7.4=22.8$ kJ/mol, with the diequatorial chair free of such strain.