A small steel ball falls through glycerine at a steady terminal speed of $2.0\ \text{cm/s}$. A second ball, of the same steel but with twice the radius, is tied to the first by a short, light thread, and the pair is released in the same glycerine with the large ball below the small one. Stokes' law holds for each ball, and each feels the same drag it would feel alone (ignore the effect of one ball on the flow round the other). What is the terminal speed of the pair? Give the answer in $\text{cm/s}$.