$a)\ \widehat{xOy}+\widehat{yOz}=180^{\circ}$ (hai góc kề bù)
$⇒ 60^{\circ}+\widehat{yOz}=180^{\circ}$
$⇒\widehat{yOz}=180^{\circ}-60^{\circ}$
$⇒\widehat{yOz}=120^{\circ}$
$b)\ \widehat{mOn}=\dfrac{1}{2}·(\widehat{xOy}+\widehat{yOz})$
$⇒ \widehat{mOn}=\dfrac{1}{2}·180^{\circ}$
$⇒ \widehat{mOn}=90^{\circ}$