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} 14\;cm\\ 10\;cm \\13\;cm \\12\;cm \end{array} $