Answer:
X=40°
X=30°
X=50°
Step-by-step explanation:
Let our unknown angles be denoted by 
Part I
We are given the sum of the angles as 70°, the known as 30° and the unknown as X;
To find X, we subtract the known angle from the sum as:
X=70°-30°=40°
Hence X= 40°
Part II
We are given the sum of the angles as 70°, the known as 40° and the unknown as X;
To find X, we subtract the known angle from the sum as:
X=70°-40°=30°
Hence X= 30°
Part III
We are given the sum of the angles as 80°, the known as 30° and the unknown as X;
To find X, we subtract the known angle from the sum as:
X=80°-30°=50°
Hence X= 50°