So a triangle has to be 180° overall And that is an equilateral triangle therefore every angle is equal so every angle inside is 60° so ?=60° another way you can find this out is a straight line is equal to 180° and since you have the external angle which equals 120° you do 180°-120° and get 60° and the 180°-60°-60°=60° and that’s your mystery angle. I hope that’s not too confusing.
