We know that : Sum of Angles in a Triangle is equal to : 180°
⇒ In ΔRST, The Sum of Angles ∠R , ∠S , ∠T should be equal to 180°
⇒ m∠R + m∠S + m∠T = 180°
⇒ (2x + 10)° + (2x + 25)° + (x - 5)° = 180°
⇒ (2x + 2x + x) + (10° + 25° - 5°) = 180°
⇒ 5x + 30° = 180°
⇒ 5x = 180° - 30°
⇒ 5x = 150°
⇒ x = 30°
