Calculate the value of the exterior angle

An exterior angle of the triangle equals to the sum opposite interior angles, so

8x+15 = (4x+5) + (3x+20)

8x +15 = 7x +25

x=10

Now, we can calculate the exterior angle.

8x+15 = 8*10 + 15 = 80+15 = 95

Answer is 95⁰.

not only in mathematics, scientific proof also are required in the other field of science. also maths proofs are of particular structure, containing steps and logics. so proving is like doing science, it is science. that's why it is important to prove. 

an inaccurate proof leads to errors, or stating results that are not accurate.  which wil mess the probelm up you dont want this