[tex]m(\ \textless \ AOB)+m(\ \textless \ BOC)=2\cdot 60=120^0\\
m(\ \textless \ AOB)+2m(\ \textless \ AOB)=120^0\\
3m(\ \textless \ AOB)=120\\
m(\ \textless \ AOB)=40^0\\
m(\ \textless \ BOC)=80^0\\
m(\ \textless \ B'OC)=180^0-m(\ \textless \ BOC)=180^0-80^0=100^0
[/tex]