Since angles A and B are complementary, we know that:
\[ m∠A + m∠B = 90° \]
Given that \( m∠A = x° \) and \( m∠B = 2x° \), we can substitute these expressions into the equation:
\[ x + 2x = 90° \]
This simplifies to:
\[ 3x = 90° \]
To find \( x \), divide both sides by 3:
\[ x = 30° \]
Now, we can find the measure of \( ∠B \):
\[ m∠B = 2x = 2 \cdot 30° = 60° \]
Therefore, the measure of \( ∠B \) is 60 degrees.