\(\begin{array}{l}
Ap\,\,dung\,\,\,dinh\,\,ly\,\,Pitago\,\,ta\,\,co:\\
BC = \sqrt {A{B^2} + A{C^2}} = \sqrt {{{15}^2} + {{20}^2}} = 25\,\,cm.\\
Ap\,\,dung\,\,ti\,\,so\,\,luong\,\,giac\,\,cua\,\,goc\,\,nhon\,\,trong\,\,tam\,\,giac\,\,vuong\,\,ta\,\,co:\\
\sin \angle B = \frac{{AC}}{{BC}} = \frac{{20}}{{25}} = \frac{4}{5}\\
\Rightarrow \angle B \approx {53^0}8'\\
\Rightarrow \angle C = {90^0} - \angle B \approx {36^0}52'.
\end{array}\)
