∵OM平分∠AOB
∴∠AOM=∠MOB=∠AOB/2
∵ON平分∠BOC
∴∠NOC=∠BON=∠BOC/2
∵∠NOC=3∠AOM
∴∠BOC/2=3∠AOB/2
∴∠BOC=3∠AOB
∵∠BON-∠MOB=20
∴∠BOC/2-∠AOB/2=20
∴∠BOC-∠AOB=40
∴3∠AOB-∠AOB=40
∴∠AOB=20
∴∠BOC=3∠AOB=60
∴∠AOC=∠AOB+∠AOC=80
∵OM平分∠AOB
∴∠AOM=∠MOB=∠AOB/2
∵ON平分∠BOC
∴∠NOC=∠BON=∠BOC/2
∵∠NOC=3∠AOM
∴∠BOC/2=3∠AOB/2
∴∠BOC=3∠AOB
∵∠BON-∠MOB=20
∴∠BOC/2-∠AOB/2=20
∴∠BOC-∠AOB=40
∴3∠AOB-∠AOB=40
∴∠AOB=20
∴∠BOC=3∠AOB=60
∴∠AOC=∠AOB+∠AOC=80