In a right-angled triangle ∆ABC, the altitude AB is 5 cm, and the base BC is 12 cm. P and Q are two points on BC such that the areas of $\triangle ABP, \triangle ABQ$ and $\triangle ABC$ are in arithmetic progression. If the area of ∆ABC is 1.5 times the area of $\triangle ABP$, the length of PQ, in cm, is