Step-by-step explanation:
[tex]360 = 60 + 4x \\ 360 - 60 = 4x \\ 300 = 4x \\ \frac{300}{4} = \frac{4x}{4} \\ x = 75[/tex]
The total internal angle of the polygon is 360°. Since there are 4 unknown angles but the same, we would just add all those to 60° to equal a full 360°.
