m(<AOB)+m(<BOC)+m(<COA)=360 grade
2x+20+3x+10+4x+15=360,scadem 45
9x=315/9
x=35
m(<AOB)=2x+20=70+20=90 grade
m(<BOC)=3x+10=105+10=115 grade
m(<COA)=4x+15=140+15=155 grade
[OM-bis<BOC=>m(<BOM)=m(<MOC)=m(<BOC)/2=115/2=57.5 grade
[ON-bis<COA=>m(<CON)=m(<NOA)=m(<COA)/2=155/2=77.5 grade
m(<MON)=m(<MOC)+m(<CON)=57.5+77.5=135 grade