In the adjoining figure $TP$ and $T'Q$ are parallel tangents to a circle of radius $r,$ with $T$ and $T'$ the points of tangency. $PT''Q$ is a third tangent with $T''$ as point of tangency. If $TP=4$ and $T'Q=9$ then $r$ is
$\frac{25}{6}$
$6$
$\frac{25}{4}$
a number other than $\frac{25}{6},$ $6,$ or
$\frac{25}{4}$
