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°.