An acyclic saturated hydrocarbon of formula $\mathrm{C}{}_7\mathrm{H}{}_{16}$ contains exactly one quaternary carbon (bonded to four carbons) and exactly one tertiary carbon; all its other carbons are primary or secondary. How many primary carbons does it contain?

Among the nine heptane isomers, only 2,2,3-trimethylbutane, (CH3)3C-CH(CH3)2, has exactly one quaternary and one tertiary carbon. It has 7 carbons: 1 quaternary, 1 tertiary, 0 secondary and so 5 primary. This matches the relation for acyclic alkanes, primary = 2 + (tertiary) + 2(quaternary) = 2 + 1 + 2 = 5.