It is stated that the sum of the interior angles of a quadrilateral is 360°. It means that:
[tex]w+x+y+z=360[/tex]
If w is the average of x, y and z, then:
[tex]w=\frac{x+y+z}{3}[/tex]
Use this information to replace w in the first equation:
[tex]\begin{gathered} \frac{x+y+z}{3}+x+y+z=360 \\ \frac{4}{3}(x+y+z)=360 \\ x+y+z=360\cdot\frac{3}{4} \\ x+y+z=270 \end{gathered}[/tex]
x+y+z=270°.
