\( 5x + 3x + 1x = 180^{\circ} \)
\( 9x = 180^{\circ} \)
\( x = \frac{180^{\circ}}{9} = 20^{\circ} \)
\( 5x = 5 \cdot 20^{\circ} = 100^{\circ} \)
Ответ: 100°