A primary (1${}^{\circ }$) carbon is bonded to only one other carbon atom. How many primary carbons are present in $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{CH}{}_2-\mathrm{CH}(\mathrm{CH}{}_3){}_2$?

Count the carbons bonded to exactly one other carbon. Each of the 5 CH3 groups is attached to a single carbon, so they are primary. The CH2 carbon is secondary, the CH carbon is tertiary and the quaternary C is bonded to four carbons. The number of primary carbons is 5.