ABCD is a quadrilateral such that $\angle D = 90^{\circ}$. A circle $(O,r)$ touches the sides $AB, \; BC,\; CD$ and $DA$ at $PQR$ and $S$ respectively. If $BC = 38\; cm, \; CD = 25\;cm$ and $BP = 27\;cm$ then $r$ = __________

Question

$\begin{array}{1 1} (a)\;14\;cm\\(b)\; 10\;cm \\(c)\;13\;cm \\(d)\;12\;cm \end{array}$