Question

A quadrilateral has three acute angles each measures 80°. What is the measure of the fourth angle?

Answer 1

$$\begin{gathered} \mathrm{Let}x\mathrm{be}\mathrm{the}\mathrm{fourth}\mathrm{angle}. \\ \mathrm{Since},\mathrm{the}\mathrm{sum}\mathrm{of}\mathrm{all}\mathrm{angles}\mathrm{of}\mathrm{a}\mathrm{quadrilateral}\mathrm{is}360°,\mathrm{we}\mathrm{have}: \\ 80°+80°+80°+x=360° \\ \Rightarrow 240°+x=360° \\ \Rightarrow x=120° \\ \therefore \mathrm{The}\mathrm{fourth}\mathrm{angle}\mathrm{is}120°. \end{gathered}$$