∵B处在A处的南偏西45°C处在A处的南偏东15°
∴∠BAC=45+15=60°
∴A处在B处的北偏东45°
∵C处在B处的北偏东80°
∴∠ABC=80-45=35°
∴∠ACB=180-35-60=85°