Answer:
70°
Explanation:
An euclidean triangle (a triangle on a straight plane) has sum of three angles equal to 180°.
When we talk about a triangle, we usually talk about an euclidean one.
There are also a non-euclidean triangles which are a triangle on circular plane which has sum of angles above 180° and one that is a hyperbolic triangle version which has sum below 180°.
Back to the question, since we know that the triangle is an euclidean one and thus has the sum equal 180°.
x+20+40+2x-30 = 180
Combine like terms:
3x+30=180
Subtract both sides:
3x+30-30=180-30
3x=150
Divide both sides by 3.
3x/3 = 150/3
x = 50
Since we want to find angle B which has measure of 2x-30, substitute x = 50 in 2x-30.
2(50)-30 = 100-30 = 70
Therefore angle B is 70°