Answer
$${81^ \circ }$$
Work Step by Step
$$\eqalign{
& \frac{{9\pi }}{{20}} \cr
& {\text{we know that 1 radian}} = \left( {\frac{{{{180}^ \circ }}}{\pi }} \right){\text{. }}\left( {{\text{see}}\,\,{\text{page 668}}} \right){\text{ Then}}{\text{,}} \cr
& \frac{{9\pi }}{{20}}{\text{radians}} = \frac{{9\pi }}{{20}}\left( {\frac{{{{180}^ \circ }}}{\pi }} \right) \cr
& {\text{multiplying}} \cr
& \frac{{9\pi }}{{20}}{\text{radians}} = \frac{{9\left( {{{180}^ \circ }} \right)}}{{20}} \cr
& \frac{{9\pi }}{{20}}{\text{radians}} = 9\left( {{9^ \circ }} \right) \cr
& \frac{{9\pi }}{{20}}{\text{radians}} = {81^ \circ } \cr} $$