A triangle needs a total of 180° to be a complete triangle. First let's solve for m∠B since we have the other angles given in that triangle.
30 + 70 = 100
180 - 100 = 80° still needed to form a complete triangle. This is what m∠B equals.
From the image provided, we can see that both triangles are the exact same. So all we need to do now is find the angle that matches m∠B. In this case that angle is m∠D. Knowing that m∠B = 80° the final answer is:
