In $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{CH}(\mathrm{CH}{}_3)-\mathrm{CH}{}_2-\mathrm{CH}{}_3$, what is the ratio of the number of primary : secondary : tertiary hydrogen atoms?

Hydrogens are classed by the carbon that bears them. The CH3 groups (3 + 1 + 1 = 5 of them) carry 15 primary H. The single CH2 carries 2 secondary H and the single CH carries 1 tertiary H. The ratio is 15 : 2 : 1, and the total is 18 = 18 H, as required for C8H18.