Given: ∠COB=(3x−10)° ∠AOC=(7x+40)° Calculation: Sum of angles on a straight line is 180° ⇒∠AOC+∠COB=180° ⇒(7x+40)°+(3x−10)°=180° ⇒10x+30°=180° ⇒10x=180°−30° ⇒x=15° ⇒∠COB=(3x−10)° ⇒(3×15−10)°=35° ∴ ∠COB = 35° The correct option is 2 i.e. 35°. The Sum of angles on a straight line is 180°.