In triangle BDE, angle B is 30% of angle D and angle E is 10 degrees greater than angle D. Find angle B.

Let’s solve the problem by writing an equation. We introduce the variable x – this will be the degree measure of the angle D. Then the other two angles will be:
∠ B = x * 30% = 0.3x;
∠ E = x + 10.
180 ° is the sum of the angles of the triangle, we make the equation:
x + 0.3x + x + 10 = 180
2,3x + 10 = 180
2,3x = 170
x = 170 / 2.3 = 73.9130.
Let’s take an integer value. We got that the angle D = 73 °.
∠ B = 0.3 * 73 = 21.9 °
We take an integer value for the answer. Of the proposed answers, a) 21 ° is suitable.
Answer: The degree measure of angle B is 21 °.