Ta vẽ như sau: 

a) Ta có Om là tia phân giác của góc xOz => $\widehat{xOm} =\widehat{xOz}:2 = 60^{\circ}:2=30^{\circ}$

b) $\widehat{yOz} +\widehat{zOx} = 180^{\circ}$ (kề bù)

=>$\widehat{yOz} = 180^{\circ}-60^{\circ}=120^{\circ}$

c) $\widehat{xOm} +\widehat{mOy} = 180^{\circ}$ (kề bù)

=> $\widehat{mOy}$ = $180^{\circ}-\widehat{xOm} =150^{\circ}$

Ta có: $\widehat{yOn} +\widehat{nOm} =\widehat{yOm}$

=> $\widehat{nOm}=\widehat{yOm}-\widehat{yOn}= 150^{\circ}-60^{\circ}=90^{\circ}$