In the arrangement as shown mB=3M , density of liquid is ρ  and density of block B is 2ρ .  The system is released from rest so that block B moves up when in liquid and moves down when out of liquid with the same acceleration.  The mass of block A is

Acceleration when block B is in liquid a1=mAg−(mBg−up  thrust)mA+3m                                                                                       =mAg−(3mg−3m2ρρg)mA+3m(↑)                     Acceleration when block B is outside the liquid is a2=3mg−mAgmA+3m(↓)                     Given a1=a2                     mAg−32mg=3mg−mAg                     2mAg=92mg                 mA=(94)m