To solve for the measure of angle \( B \):
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Since angles \( A \) and \( B \) are complementary, we have: \[ m<A + m<B = 90^\circ \] Given \( m<A = x \) and \( m<B = 2x \), we can substitute these values: \[ x + 2x = 90^\circ \] \[ 3x = 90^\circ \] \[ x = 30^\circ \]
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Now, we can find the measure of angle \( B \): \[ m<B = 2x = 2(30^\circ) = 60^\circ \]
Thus, the measure of angle \( B \) is \( \boxed{60} \).