In the adjoining figure, I and II are circles with P and Q as centres respectively, The two circles touch each other and have a common tangent that touches them at points R and S respectively. This common tangent meets the line joining P and Q at O. The diameters of I and II are in the ratio of 4 : 3. It is also known that the length of PQ is 28 cm.
What is the ratio of the length of PQ to that of QO?