Since angles A and B are complementary, we have:
\[ m∠A + m∠B = 90° \]
Given:
\[ m∠A = x° \quad \text{and} \quad m∠B = (2x)° \]
Substituting these values into the relationship for complementary angles, we get:
\[ x + 2x = 90° \]
This simplifies to:
\[ 3x = 90° \]
Now, solving for \(x\):
\[ x = \frac{90°}{3} = 30° \]
Now, we can find the measure of angle B:
\[ m∠B = 2x = 2(30°) = 60° \]
So, the measure of angle B is:
60°.