Ta có: $\frac{1}{5}\widehat{xOz}=\frac{1}{4}\widehat{yOz}$ suy ra $\widehat{xOz}=\frac{5}{4}\widehat{yOz}$
Do hai góc xOz và yOz là hai góc kề nhau nên $\widehat{xOz}+ \widehat{yOz}=\widehat{xOy}=90^{\circ}$
Do đó $\frac{5}{4}\widehat{yOz}+\widehat{yOz}=\frac{9}{4}\widehat{yOz}=90^{\circ}$ hay$ \widehat{yOz}=40^{\circ}$
Suy ra $\widehat{xOz}= \frac{5}{4}^40{\circ}=50^{\circ}$