Two comparable triangles ABC, PQR have perimeters of 64 cm and 24 cm, respectively. Find the length of AB, if PQ = 12 cm.

We are aware that the ratio of similar triangles' perimeters to their corresponding sides is the same.

$\therefore \frac{\text{ Perimeter of }∆ABC}{\text{ Perimeter of }∆PQR}=\frac{AB}{PQ}$

$\frac{64}{24}=\frac{AB}{12}$

$AB=\frac{64\times 12}{24}=32 \mathrm{cm}$