\(\mu = \frac{u}{v} = \frac{x}{v}\)\( = \frac{x}{6}\) .....\((i)\)

Now when viewed from face \((2)\)

\(\mu = \frac{{15 - x}}{v} = \frac{{15 - x}}{4}\) .....\((ii)\)

From equation \((i)\) and \((ii)\)

\(\mu = \frac{{15 - 6\mu }}{4}\)\( \Rightarrow \mu = 1.5\).