Answer
The mass of block B is $~~3.3~kg$
Work Step by Step
Since block A slides at a constant speed, the forces pulling the system to the right (up the incline) must be equal in magnitude to the force pulling the system to the left (down the incline).
We can find $m_B$:
$m_B~g+m_A~g~\mu_k~cos~\theta = m_A~g~sin~\theta$
$m_B+m_A~\mu_k~cos~\theta = m_A~sin~\theta$
$m_B = m_A~sin~\theta - m_A~\mu_k~cos~\theta$
$m_B = (10~kg)~sin~30^{\circ} - (10~kg)~(0.20)~cos~30^{\circ}$
$m_B = 3.3~kg$
The mass of block B is $~~3.3~kg$
